One plunges head first into water (5)
I believe the answer is:
diver
'one plunges head first into water' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for diver that I've seen before include "Competitor in pool" , "Cheating footballer" , "Underwater worker" , "Person operating under water" , "Grebe maybe" .)