Cousin of a herring (4)
I believe the answer is:
shad
'cousin of a herring' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for shad that I've seen before include "Herring-like fish" , "Fish related to the herring" , "Fish; dash (anag.)" , "fish that's been caught" .)