Mephisto 3398 – In the middle of the middle

For a John Grimshaw, this puzzle was not too bad; only of medium difficulty, with some easier starter clues. But this is still Mephisto, and it’s highly like that most solvers have not heard of the long words around the edge.
Note: This week’s Mephisto by Paul McKenna is very easy, so if you’ve been wanting to try one, now is your chance.