Deep swimmer (4)
I believe the answer is:
bass
I believe this clue is a double definition.
'deep' is the first definition.
(I know that deep can be written as bass)
'swimmer' is the second definition.
(I know that bass is a type of fish)
(Other definitions for bass that I've seen before include "Fish for the low-voiced" , "Fish, guitar" , "Low fellow" , "Large fish - singer" , "Performer" .)