Head back with a swimmer (4)
I believe the answer is:
tuna
'swimmer' is the definition.
(tuna is a kind of aquatic vertebrate)
'head back with a' is the wordplay.
'head' becomes 'nut' (informal term for somebody's head**).
'back' shows that the letters should be reversed in order.
'with' is a charade indicator (letters next to each other).
'nut' backwards is 'tun'.
'tun'+'a'='TUNA'
(Other definitions for tuna that I've seen before include "Bluefin, e.g." , "Marine food fish" , "Widely-eaten fish" , "Large salt water fish" , "Large fish found in a nut" .)