Sign to return for Verne's captain (4)
I believe the answer is:
nemo
'verne's captain' is the definition.
(I've seen this in another clue)
'sign to return' is the wordplay.
'sign' becomes 'omen' (omen is a kind of sign).
'to return' says the letters should be written in reverse.
'omen' reversed gives 'NEMO'.
'for' is the link.
(Other definitions for nemo that I've seen before include "Finding -, animated film" , "Fish sought in film" , "- me impune lacessit (motto of Scotland)" , "Captain of the Nautilus" , "Jules Verne's submarine captain" .)