ACROSS
1 Shakespearean counsel to wit: time to interject last word (8)
VIZAMENT – VIZ [to wit] + T(ime), with AMEN [last word] “interjected”
5 Wife in charge probes tattoo curl with a glance (6)
INWICK – W(ife) I(n)/C(harge) “probes” INK [tattoo]. This is a glancing shot in the game of curling, not any kind of withering look!
8 Environmental activists nipping over gate (3)
XOR – X(tinction) R(ebellion) “nipping” O(ver). Exclusive-OR logic gate.
9 26 queens played in no-trump card game (10)
LANSQUENET – LA [= LAH] + (QUEENS*) in N(o)T(rump)
10 Fruits of those treated by bloodsuckers? (8)
LEECHEES – double def. Another spelling of lychees/litchis, or else possibly ones being leeched by metaphorical leeches using actual leeches.
11 Perhaps visiting Madrid less one is forcibly put to work (6)
INSPAN – IN SPA{i}N
12 Chart to help calculating two sides of triangle with generically labelled vertices? (4)
ABAC – if a triangle’s points are A, B and C, two of its sides will be AB + AC.
14 Contribute with starter of nuts, as in dwarf chestnut (10)
CHINQUAPIN – CHIP IN, with N{uts} + QUA [as] in
Yen for introduction of vernacular swearing, half-lost in essence (5-5)
YLANG-YLANG – {s-Y}LANGY [vernacular (with Y replacing its first letter)] + LANG{uage} [swearing, “half-lost”]
20 Endlessly extract sticky substance from small Chilean rodent (4)
DEGU – DE-GU{m}
23 House-sitter keeps for one Italian the vehicle used by Turner (6)
MEGILP – MP [sitter in the House (of Commons)] “keeps” E.G. IL [for one | Italian the]. Megilp is not a roadworthy vehicle, but a medium used in oil painting.
24 Some current person almost catches island salamander in Alabama (8)
AMPHIUMA – AMP [some current] + HUMA{n} [person “almost”] “catches I(sland)
25 An ornamental sample of daguerreotypy RA can’t hang (10)
PYRACANTHA – hidden in {daguerroty}PY RA CAN’T HA{ng}
26 What’s common in flatfish and loach is a component in scales (3)
LAH – take all the letters held in common by {f}LA{tfis}H and L{o}A{c}H. Lah is the “component in scales” that falls between so and ti.
27 Recalled what might be spiritual exists, knowing it? (6)
GNOSIS – reversed SONG [what might be spiritual] + IS [exists], semi-&lit
28 Piece of shareware wanting source for desktop material? (4-4)
REWA-REWA – hidden in {sha}REWA-RE WA{nting}. A tree in NZ, used in furniture making.
DOWN
1 Standard-bearer’s chagrin with mountaineer missing summit (9)
VEXILLARY – VEX [chagrin] + (Edmund) {h}ILLARY
2 A porter, maybe a Zulu, set up camp with defences (7)
ZAREEBA – reverse all of A BEER [porter, maybe] A Z(ulu). Originally a Sudanese word, apparently.
3 Whisky, heroin addict’s foremost dark, thick substance (6)
MALTHA – MALT [whisky] + H(eroin) + A{ddict}. Mineral tar.
4 Possibly chamber music auditorium boosts bottom line, unlikely to make a killing (9)
NONLETHAL – take a NONET HALL [possibly chamber music auditorium], and “boost” the bottom L to higher up in the answer.
5 I answer question about 13th and 16th letters in Arctic language (7)
INUPIAQ – I A(nswer) Q(uestion) about NU + PI [13th and 16th letters, in the Greek alphabet]
6 Turning four leading bard would like cradle in treetop? (4-5)
WIND-SHAK’D – WIND [turning] + SHAK{espeare} [four (letters) leading bard] + ‘D [world]
7 French sectarians releasing pent-up emotions, never having succeeded (7)
CATHARI – CATHAR{s}I{s} [releasing pent-up emotions, with all instances of the letters S (= succeeded) removed]
13 John, war representative, carrying combat jacket over docks in Texas (9)
CANAIGRES – CAN [John] + ARES [(divine) war representative], “carrying” reversed GI [combat jacket (in judo)]. Docks as in the plants, here.
15 Linear measure not applicable with lack of rhythmic pattern (9)
NANOMETRE – N/A with NO METRE
16 Rocket launchers needing uniform speed of sound for southern place where sea battles were enacted
NAUMACHIA – take NASA [rocket launchers], and replace S(outhern) with U MACH-1 [uniform | speed of sound]
18 Be a burden for docked horse after a place (3,4)
LIE UPON – PON{y} after LIEU [a place]
19 S American opossums from heart of Guyana pits (7)
YAPOCKS – {gu}YA{na} + POCKS [pits]. Yapocks are adorable by the way, such big hands!
21 With note repeated at top, middle and bottom, select bowl with two handles (7)
ECUELLE – {E}CU{E}LL{E} – CULL is select; introduce the note E in the three places mentioned.
22 Somehow had to protect voter against mounting spiritual contemplation (6)
DHYANA – (HAD*) to “protect” reversed NAY [voter against]
Thank you blogger and thank you setter for a fine example of the MCS at its best