Turn sign around for submarine captain (4)
I believe the answer is:
nemo
'for submarine captain' is the definition.
(character from 20,000 Leagues Under the Sea)
'turn sign' is the wordplay.
'turn' shows that the letters should be reversed in order.
'sign' becomes 'omen' (omen is a kind of sign).
'omen' backwards is 'NEMO'.
'around' acts as a link.
(Other definitions for nemo that I've seen before include "- me impune lacessit (motto of Scotland)" , "Jules Verne's submarine captain" , "Fish sought in film" , "Literary captain" , "Captain of the Nautilus" .)