A fanatic over fish (4)
I believe the answer is:
tuna
'fish' is the definition.
(tuna is a type of fish)
'a fanatic over' is the wordplay.
'fanatic' becomes 'nut' (I've seen this before).
'over' shows that the letters should be reversed in order.
'a'+'nut'='anut'
'anut' in reverse letter order is 'TUNA'.
(Other definitions for tuna that I've seen before include "(Meat of) large fish" , "Species of fish" , "Common food fish" , "Popular food fish" , "Widely-eaten fish" .)