Cover up topless swimmer (3)
I believe the answer is:
ide
'swimmer' is the definition.
(type of fish, something that swims)
'cover up topless' is the wordplay.
'cover up' becomes 'hide' (I've seen this before).
'topless' means to remove the first letter.
'hide' with its initial letter taken away is 'IDE'.
(Other definitions for ide that I've seen before include "Fish, the orfe" , "Fish of the carp family" , "Silver orfe" .)