FOI was the chestnut 9ac, but few of the other clues much sense of deja vu about them. In fact I noted a lot of fun things during/after my solve: the “revolutionary essence” at 1ac, and inventive cluing at 16ac, 19ac, 4dn etc. I think above-average cluing ingenuity combined with definition parts that, as mentioned above, were very rarely amenable to buffing, made this a good chewy end-of-week challenge, two thumbs up to the setter from this blogger despite the lamentable lack of TLS-iness. COD to 13dn for the great economy of the cryptic part combined with the definition’s circumlocution above and beyond the call of duty.
One last thing to mention is that there was a definite northernish flavour here from the tasty combination of 6dn and 28ac – I like to think that 
myrtilus000 will have been able to fill out much of this puzzle just by glancing down at his breakfast plate!
ACROSS
1 Revolutionary essence derived from turning over leaf (5)
FOLIO – OIL OF [revolutionary essence | derived from], reversed by “turning over”. I was confused for a long while about why a second reversal indicator was needed, but this morning I realise that oil is a revolutionary essence because it keeps the wheels going round smoothly…
4 Play down what’s frequently pinched by lapsed criminal (4-5)
SOFT-PEDAL – OFT [frequently] “pinched by” (LAPSED*) [“criminal”]
9 Old monitor’s spoken of finance office (9) (4)
EXCHEQUER – homophone of EX CHECKER [old | monitor, “spoken of”]. FOI.
10 Believer in return of a former ruler (5)
RASTA – A TSAR [a | former ruler], reversed (probably a semi-&lit too, as pointed out)
11 Updated method backed by fighting force (6)
MODERN – MODE [method] backed by RN [fighting force]
12 Germanic aggressor once shot during stay at hospital (8)
VISIGOTH – GO [shot] during VISIT [stay], at H [hospital]
14 She’s on set, maybe to KO a couple of staff (10)
STUNTWOMAN – STUN TWO MAN [to KO | a couple of | staff]. LOI.
16 Implement not small enough? (4)
TOOL – TOO L = TOO LARGE = not small enough
19 What distinguishes regulated from deregulated crossing (4)
NODE – “regulated” differs from “deregulated” because it has NO “DE”
20 Reclusive elector’s ID defective (10)
CLOISTERED – (ELECTOR’S ID*) [“defective”]
22 Gave up supporting English after recurrent arguments (8)
FORSWORE – FOR [supporting] + E [English] after reversed ROWS [“recurrent” arguments]
23 Setback for both Irish and East European competitors (6)
RIVALS – reverse both IR [Irish] and SLAV [East European]
26 Treatment from stylist, extremely clever devil (5)
CRIMP – C{leve}R + IMP [devil]
27 Preacher‘s an active type, full of charm (9)
GOSPELLER – GOER [an active type], full of SPELL [charm]
28 Dairy product made by prudent race (4,5)
SAGE DERBY – SAGE [prudent] + DERBY [race]. Not a cheese I’m very familiar with but it looks very distinctive, a kind of marbled green effect!
29 Colour of any two short bananas (5)
TAWNY – (ANY TW{o}*) [“bananas”]
DOWN
1 Man with secret order to release mother and child (9)
FREEMASON – FREE MA + SON [release | mother (and) child]
2 Top copper’s held up, that’s clear (5)
LUCID – LID [top] in which CU [copper] is held up
3 Make too much of frequency of deliveries (8)
OVERRATE – In cricket, an OVER RATE could be the frequency of deliveries
4 Comfortable, as Carol should be after change of heart
SNUG – a carol should be SUNG, swap the middle letters.
5 Chemical from laminate child regularly picked off (6,4)
FORMIC ACID – FORMICA [laminate] + C{h}I{l}D
6 Standard family fare at teatime? (6)
PARKIN – PAR KIN [standard | family]
7 After course, my side’s bearing no shame (9)
DISHONOUR – after DISH [course], OUR [my side’s] “bearing” NO
8 Restraint for dog while a shopper’s absorbed (5)
LEASH – hidden in {whi}LE A SH{upper}
13 Party received standard warning of potential takeover bid? (5,5)
JOLLY ROGER – JOLLY [party] + ROGER [received]. The definition part refers to the standard, as in flag, that warns seagoers that they may be about to be boarded and taken over by pirates.
15 Limited stress good for junior (9)
UNDERLING – UNDERLIN{e} [“limited” stress] + G [good]
17 In the course of last year, foolish daughter’s seduced (3,6)
LED ASTRAY – in (LAST YEAR*) [“foolish”], D for daughter
18 Vociferous march by conservationists (8)
STRIDENT – STRIDE [march] by NT [conservationists]
21 Stolen goods ultimately disposed of (6)
SWIPED – {good}S + WIPED [disposed of]
22 Defendant’s third book essential in court case (5)
FACTS – {de}F{endant} + ACTS [book]
24 Radiant silver-blue (5)
AGLOW – AG LOW [silver | blue]
25 Grey area reserved (4)
ASHY – A SHY [area | reserved]
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