Fictional captain (4)
I believe the answer is:
nemo
'fictional captain' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nemo that I've seen before include "Fish sought in film" , "Jules Verne's submarine captain" , "Literary captain" , "Captain of the Nautilus" , "- me impune lacessit (motto of Scotland)" .)