Swimmer in voice? (4)
I believe the answer is:
bass
This clue is a double definition.
'swimmer' is the first definition.
(bass is a kind of fish)
'voice?' is the second definition.
(I know that bass is a type of voice)
'in' acts as a link.
(Other definitions for bass that I've seen before include "Lowest male singer" , "Fish (which sings?)" , "Deep-toned" , "Fish; male singing voice" , "Fish, guitar" .)