Samovar -
22D involves taking the 'unveiled plans' (3 letters) and using them to 'pen' (O is the extra letter) a poison. When you do so you will get a synonym for Spartan - and a revelation about what's going on in the 'cells available'.
21D is another of the same kind - and C is the extra letter. Remember it's the 'overthrow' of moderation you need, not just moderation.
I've solved all the clues apart from 7dn and 24ac. Hints welcome!
I'm pretty sure the setter has slipped up with the clue to 10dn, which needs to generate an extra C but then leaves the wrong letters for the entry.
But the extra letters spell an instruction that I can see no way of using. Giving up for now.