Sailor on rough sea becomes humble (5)
I believe the answer is:
abase
'humble' is the definition.
(synonyms)
'sailor on rough sea' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'on' means one lot of letters goes inside another.
'rough' indicates anagramming the letters (a rough or broken ordering of the letters).
'sea' is an anagram of 'ase'.
'ab' going within 'ase' is 'ABASE'.
'becomes' is the link.
(Other definitions for abase that I've seen before include "Humiliate" , "Lower" , "what 11 [REPRIMANDS] are meant to do" , "put to shame" , "Degrade (oneself)" .)