Fish, a freak doing the backstroke? (4)
I believe the answer is:
tuna
'fish' is the definition.
(tuna fish is a kind of fish)
'a freak doing the backstroke?' is the wordplay.
'freak' becomes 'nut' (I've seen this before).
'doing the backstroke?' says the letters should be written backwards.
'a'+'nut'='anut'
'anut' back-to-front is 'TUNA'.
(Other definitions for tuna that I've seen before include "Large food fish" , "Widely-eaten fish" , "Sea creature" , "Marine food fish" , "Species of fish" .)