Swimmer, a fanatic doing the backstroke? (4)
I believe the answer is:
tuna
'swimmer' is the definition.
(tuna is a kind of aquatic vertebrate)
'a fanatic doing the backstroke?' is the wordplay.
'fanatic' becomes 'nut' (I've seen this before).
'doing the backstroke?' says the letters should be written backwards.
'a'+'nut'='anut'
'anut' reversed gives 'TUNA'.
(Other definitions for tuna that I've seen before include "swimmer" , "Marine food for your strange aunt" , "Large food fish" , "Common food fish" , "Sea creature" .)