Time to defeat swimmer (5)
I believe the answer is:
trout
'swimmer' is the definition.
(trout is a kind of aquatic vertebrate)
'time to defeat' is the wordplay.
'time' becomes 't'.
'to defeat' becomes 'rout' (routing is a kind of defeating).
't'+'rout'='TROUT'
(Other definitions for trout that I've seen before include "Rainbow, say" , "tickled to death?" , "Tasty fish, brown or rainbow" , "Oily fish" , "Swimmer" .)