Sign submarine captain will return (4)
I believe the answer is:
omen
'sign' is the definition.
(I know that omen is a type of sign)
'submarine captain will return' is the wordplay.
'submarine captain' becomes 'nemo' (character from 20,000 Leagues Under the Sea).
'will return' says the letters should be written backwards.
'nemo' in reverse letter order is 'OMEN'.
(Other definitions for omen that I've seen before include "Forecast" , "Forewarning, harbinger" , "Portent of evil" , "Prognostication" , "Gloomy sign" .)