Fish an enthusiast put back (4)
I believe the answer is:
tuna
'fish' is the definition.
(tuna is a type of fish)
'an enthusiast put back' is the wordplay.
'an' becomes 'a'.
'enthusiast' becomes 'nut' (nut can mean an enthusiast or fan).
'put back' says the letters should be written backwards.
'a'+'nut'='anut'
'anut' reversed gives 'TUNA'.
(Other definitions for tuna that I've seen before include "Marine food for your strange aunt" , "Fish; edible cactus" , "Popular food fish" , "Large sea-fish" , "Widely-eaten fish" .)