Sarcastic expert assembled crib (7)
I believe the answer is:
acerbic
'sarcastic' is the definition.
(acerbic means sarcastic or sharp)
'expert assembled crib' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'assembled' indicates an anagram.
'crib' with letters rearranged gives 'rbic'.
'ace'+'rbic'='ACERBIC'
(Other definitions for acerbic that I've seen before include "Cutting" , "Sharp, forthright" , "Bitter-tasting" , "Astringent; sour" , "tart" .)