Criticise choice under fool (7)
I believe the answer is:
nitpick
'criticise' is the definition.
(nitpicking is a kind of criticising)
'choice under fool' is the wordplay.
'choice' becomes 'pick' ('pick' can be a synonym of 'choice').
'under' means one lot of letters go next to another.
'fool' becomes 'nit' (I've seen this before).
'pick' after 'nit' is 'NITPICK'.
(Other definitions for nitpick that I've seen before include "Have little complaint" , "Be fussy and pedantic" , "Be fussily pedantic" , "Raise petty objections" , "Make petty criticism" .)