Indication submariner is surfacing (4)
I believe the answer is:
omen
'indication' is the definition.
(I've seen this in another clue)
'submariner is surfacing' is the wordplay.
'submariner' becomes 'Nemo' (Captain Nemo in 20,000 Leagues Under the Sea).
'is surfacing' shows that the letters should be reversed in order (as opposed to capsizing).
'nemo' in reverse letter order is 'OMEN'.
(Other definitions for omen that I've seen before include "Portent of things to come" , "Sign of something to come" , "Harbinger, portent" , "Harbinger, prophetic sign" , "Event regarded as a prophetic sign of good or evil" .)