Lost fish (4)
I believe the answer is:
nemo
'lost fish' is the definition.
(from the film Finding Nemo)
This is all the clue.
(Other definitions for nemo that I've seen before include "commander for the Nautilus" , "- me impune lacessit (motto of Scotland)" , "Literary captain" , "Jules Verne's submarine captain" , "Jules Verne captain" .)