The first defeat for a swimmer (5)
I believe the answer is:
trout
'for a swimmer' is the definition.
(trout is a kind of aquatic vertebrate)
'the first defeat' is the wordplay.
'the first' becomes 't' (rst letter of 'the').
'defeat' becomes 'rout' (routing is a kind of defeating).
't'+'rout'='TROUT'
(Other definitions for trout that I've seen before include "one often tickled?" , "interfering old woman" , "It rises to fly" , "old bag" , "bad-tempered woman" .)