Shelter for backward swimmer (3)
I believe the answer is:
lee
'shelter' is the definition.
(as in lee side, the side sheltered from wind)
'backward swimmer' is the wordplay.
'backward' says the letters should be written in reverse.
'swimmer' becomes 'eel' (eel is a type of fish).
'eel' back-to-front is 'LEE'.
'for' is the link.
(Other definitions for lee that I've seen before include "Confederate general" , "The sheltered side of something" , "Towards the side away from the wind (naut)" , "Sheltered from the wind in Cork, perhaps" , "actor" .)