Forecast enigmatic sea captain backed (4)
I believe the answer is:
omen
'forecast enigmatic sea' is the definition.
'omen' can be an answer for 'forecast' (synonyms). I am not sure about the 'enigmatic sea' bit.
'captain backed' is the wordplay.
'captain' becomes 'nemo' (character from 20,000 Leagues Under the Sea).
'backed' shows that the letters should be reversed in order.
'nemo' backwards is 'OMEN'.
Can you help me to learn more?
(Other definitions for omen that I've seen before include "Warning token" , "Sign of things to come" , "Sign of something to come" , "Auspice" , "Warning - sign" .)