Swimmer loses head in defeat (4)
I believe the answer is:
rout
'defeat' is the definition.
(rout is a kind of defeat)
'swimmer loses head' is the wordplay.
'swimmer' becomes 'trout' (trout is a kind of aquatic vertebrate).
'loses head' means to remove the first letter.
'trout' with its first letter removed is 'ROUT'.
'in' is the link.
(Other definitions for rout that I've seen before include "Whitewash" , "Defeat comprehensively" , "Completely defeat" , "Overwhelming defeat or disorderly retreat" , "Defeat utterly" .)