Cast for small fish (5)
I believe the answer is:
sling
'cast' is the definition.
(I've seen this before)
'small fish' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'fish' becomes 'ling' (ling is a kind of fish).
's'+'ling'='SLING'
'for' acts as a link.
(Other definitions for sling that I've seen before include "throwing implement like catapult" , "Bandage support; hurl" , "Arm bandage; hurl" , "Keeping afloat" , "Injured-arm support" .)