Fish worker, an expert (3,4)
I believe the answer is:
dab hand
'an expert' is the definition.
(I've seen this before)
'fish worker' is the wordplay.
'fish' becomes 'dab' (I've seen this before).
'worker' becomes 'hand' (hand is a kind of worker).
'dab'+'hand'='DAB HAND'
(Other definitions for dab hand that I've seen before include "Past master" , "Barely wash" , "An informal expert such as band had" , "cursory wash?" .)