Sign boat captain (retd) (4)
I believe the answer is:
'sign' is the definition.
(omen is a kind of sign)
'boat captain retd' is the wordplay.
'boat captain' becomes 'Nemo' (Captain Nemo in 20,000 Leagues Under the Sea).
'retd' says the letters should be written in reverse (retired - retire can mean to go back).
'nemo' in reverse letter order is 'OMEN'.
(Other definitions for omen that I've seen before include "Event regarded as a prophetic sign of good or evil" , "Forecast" , "Portent of evil" , "Auspice" , "Sign of a future event, good or bad" .)