Echoing submarine captain’s signal (4)
I believe the answer is:
omen
'signal' is the definition.
(I know this)
'echoing submarine captain's' is the wordplay.
'echoing' shows that the letters should be reversed in order (echo can mean to reflect or bounce back).
'submarine captain' becomes 'nemo' (character from 20,000 Leagues Under the Sea).
'nemo' back-to-front is 'OMEN'.
(Other definitions for omen that I've seen before include "Harbinger, portent" , "Sign of something to come" , "Augury, portent" , "Warning - sign" , "Foreshadow" .)