Command on a submarine (4)
I believe the answer is:
dive
'command on a submarine' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dive that I've seen before include "Plunge into - disreputable club" , "Plunge head first" , "Leap into water" , "Plunge headfirst" , "Submarine captain's command" .)