Collection of letters from such a keen swimmer (4)
I believe the answer is:
hake
'swimmer' is the definition.
(type of fish)
'collection of letters from such a keen' is the wordplay.
'collection of letters from' indicates the answer is hidden within the clue.
'HAKE' can be found hidden inside 'such a keen'.
(Other definitions for hake that I've seen before include "Food fish; rhymes with create" , "Cod-like fish" , "Swimmer" , "Food fish related to the cod" , "Type of fish" .)