Frogman (5)
I believe the answer is:
diver
'frogman' is the definition.
(I know that frogman can be written as diver)
This is all the clue.
(Other definitions for diver that I've seen before include "Competitor in pool" , "Drive (anag.)" , "Frogman" , "Grebe maybe" , "Underwater worker" .)