Sign of submariner’s return (4)
I believe the answer is:
omen
'sign' is the definition.
(omen is a kind of sign)
'submariner's return' is the wordplay.
'submariner' becomes 'Nemo' (Captain Nemo in 20,000 Leagues Under the Sea).
'return' is a reversal indicator.
'nemo' back-to-front is 'OMEN'.
'of' is the link.
(Other definitions for omen that I've seen before include "Sign of something about to happen" , "Sign of likely future event" , "Sign of something to come" , "Forewarning, harbinger" , "Prophetic significance" .)