Somehow traps fish (5)
I believe the answer is:
sprat
'fish' is the definition.
(I know that sprat is a type of herring)
'somehow traps' is the wordplay.
'somehow' indicates anagramming the letters.
'traps' anagrammed gives 'SPRAT'.
(Other definitions for sprat that I've seen before include "Traps (anag.)" , "one on a fat-free diet" , "little fish" , "Herring of the east North Atlantic" , "Small edible fish" .)