Drug, not one for eater of fish (5)
I believe the answer is:
heron
'eater of fish' is the definition.
(bird which eats fish)
'drug not one' is the wordplay.
'drug' becomes 'heroin' (heroin is a kind of drug).
'not' is a deletion indicator.
'one' becomes 'i' (Roman numeral).
'heroin' with 'i' taken out is 'HERON'.
'for' is the link.
(Other definitions for heron that I've seen before include "Grey or white long-legged wading bird" , "Wader" , "Rhone (anag) - bird" , "Large British wading bird" , "flier" .)