crackabilly I can help with 13a.
old word for busybody in Chambers. Think of 'a tic' being taken out of that word and 'g' is the extra letter as I'm sure you know from the instruction.
Unfortunately I have one clue not solved and that happens to be 20a which I have as 'OU?NS' . The 'U' comes from 15d which is 'LAUGH' (Spenser word for deride). Haven't quite got all the instruction as yet but still working on it. Perimeter is also still puzzling me!