Biting witticism about right and wrong (7)
I believe the answer is:
mordant
'biting' is the definition.
(I've seen this before)
'witticism about right and wrong' is the wordplay.
'witticism' becomes 'mot' (as in a bon mot).
'about' is an insertion indicator.
'right' becomes 'r' (common abbreviation).
'wrong' is an anagram indicator.
'r'+'and'='rand'
'rand' anagrammed gives 'rdan'.
'mot' enclosing 'rdan' is 'MORDANT'.
(Other definitions for mordant that I've seen before include "Corrosive" , "Acerbic" , "Caustic" , "pungent" , "Biting, scathing" .)