Crazy person going back to New York for fish (5)
I believe the answer is:
tunny
'for fish' is the definition.
(tunny is a kind of fish)
'crazy person going back to new york' is the wordplay.
'crazy person' becomes 'nut' (nut can informally mean an insane person).
'going back' shows that the letters should be reversed in order.
'to new york' becomes 'ny'.
'nut' in reverse letter order is 'tun'.
'tun'+'ny'='TUNNY'
(Other definitions for tunny that I've seen before include "Large marine fish" , "A tuna" , "Warm water food fish" , "Common food fish" , "Marine fish, the bluefin" .)