Humble communication on boat (10)
I believe the answer is:
submissive
'humble' is the definition.
('submissive' can be similar in meaning to 'humble')
'communication on boat' is the wordplay.
'communication' becomes 'missive' (missive is a kind of communication**).
'on' means one lot of letters go next to another.
'boat' becomes 'sub' (I've seen this before).
'missive' after 'sub' is 'SUBMISSIVE'.
(Other definitions for submissive that I've seen before include "Meekly obedient" , "tendering resignation?" .)