Crossword Genius

A fanatic over fish (4)

Ross

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" .)

I've seen this clue in The Telegraph.
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