Adverse criticism? Not as a rule (9)
I believe the answer is:
exception
'adverse criticism?' is the definition.
(thesaurus)
'not as a rule' is the wordplay.
I cannot really understand how this works, but
'not' could be 'o' (I've seen this in other clues) and 'o' is present in the answer.
'as' could be 'in' (I've seen this in other clues) and 'in' is found within the remaining letters.
The remaining letters 'except' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for exception that I've seen before include "what's against the rules" , "Something not conforming to a rule" , "breach of rule" , "Omission" , "The one that proves the rule" .)