Swimmer finished poetically, clinching races (5)
I believe the answer is:
otter
'swimmer' is the definition.
(I have seen 'Good swimmer' mean 'otter' so perhaps 'swimmer' could also mean 'otter')
'finished poetically clinching races' is the wordplay.
'finished poetically' becomes 'oer' (poetic spelling of 'over').
'clinching' indicates putting letters inside.
'races' becomes 'tt' (Isle of Man Tourist Trophy races).
'oer' enclosing 'tt' is 'OTTER'.
(Other definitions for otter that I've seen before include "Water creature" , "furry creature" , "bad-tempered little beast" , "Fish-eating mammal" , "Aquatic carnivore" .)