Swimmer's more fashionable without a top (5)
I believe the answer is:
otter
'swimmer's' is the definition.
(I have seen 'Good swimmer' mean 'otter' so perhaps 'swimmer' could also mean 'otter')
'more fashionable without a top' is the wordplay.
'more fashionable' becomes 'hotter'.
'without a top' means to remove the first letter (in a down clue, top can mean the top letter).
'hotter' with its initial letter removed is 'OTTER'.
(Other definitions for otter that I've seen before include "Playful aquatic animal of 'Tarka' fame" , "River animal" , "eg Tarka" , "Freshwater brown carnivore" , "Freshwater mammal with dark brown fur" .)