Submarine's first captain comes up to deliver warnings (5)
I believe the answer is:
omens
'warnings' is the definition.
(I've seen this before)
'submarine's first captain comes up to' is the wordplay.
'first' indicates taking the first letters.
'captain' becomes 'nemo' (character from 20,000 Leagues Under the Seas).
'comes up to' says the letters should be written backwards.
The first letter of 'submarines' is 's'.
's'+'nemo'='snemo'
'snemo' written backwards gives 'OMENS'.
'deliver' is the link.
(Other definitions for omens that I've seen before include "Signs or warnings for the future" , "Prophetic signs of good or evil" , "Augurs" , "Prognostications" , "Harbingers, portents" .)