Captain in submarine movies (4)
I believe the answer is:
nemo
'captain' is the definition.
(character from 20,000 Leagues Under the Sea)
'in submarine movies' is the wordplay.
'in' indicates the answer is hidden within the clue.
'NEMO' is hidden in the letters of 'submarine movies'.
(Other definitions for nemo that I've seen before include "Finding -, animated film" , "commander for the Nautilus" , "- me impune lacessit (motto of Scotland)" , "Fish sought in film" , "Captain brought to book" .)