Average fish, shortened (3)
I believe the answer is:
par
'average' is the definition.
(thesaurus)
'fish shortened' is the wordplay.
'fish' becomes 'parr' (I've seen this before).
'shortened' means to remove the last letter.
'parr' with its final letter taken off is 'PAR'.
(Other definitions for par that I've seen before include "Usual score at golf" , "Expected number of strokes for hole (golf)" , "normal amount" , "Expected golf hole score" , "Standard amount" .)