Small herring (5)
I believe the answer is:
sprat
'small herring' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sprat that I've seen before include "Small edible fish" , "Small sea fish" , "Parts of a very small fish" , "Parts (anag.)" , "Small fish (used to catch mackerel?)" .)