Submarine's first captain comes back to make threats (5)
I believe the answer is:
omens
'make threats' is the definition.
The definition and answer can be both related to communication as well as being plural nouns.
Perhaps you can see an association between them that I don't see?
'submarine's first captain comes back' is the wordplay.
'submarine's first' becomes 's' (1st letter of 'submarine').
'captain' becomes 'nemo' (character from 20,000 Leagues Under the Sea).
'comes back' says the letters should be written backwards.
's'+'nemo'='snemo'
'snemo' reversed gives 'OMENS'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for omens that I've seen before include "Prophetic signs of good or evil" , "Harbingers, portents" , "Signs of things to come" , "Signs or warnings for the future" , "Portents, prophetic signs" .)