Sarcastic expert used crib (7)
I believe the answer is:
acerbic
'sarcastic' is the definition.
(I've seen this in another clue)
'expert used crib' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'used' is an anagram indicator.
'crib' anagrammed gives 'rbic'.
'ace'+'rbic'='ACERBIC'
(Other definitions for acerbic that I've seen before include "Astringent; sour" , "Sarcastic" , "Tasting sour or caustic in speech" , "Sour; sharp" , "Tasting bitter; forthright" .)