Humble sailor on a rough sea (5)
I believe the answer is:
abase
'humble' is the definition.
(synonyms)
'sailor on a rough sea' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'on' is an insertion indicator.
'a rough' indicates anagramming the letters (a rough or broken ordering of the letters).
'sea' anagrammed gives 'ase'.
'ab' placed inside 'ase' is 'ABASE'.
(Other definitions for abase that I've seen before include "Humble (oneself)" , "Cast down" , "what 11 [REPRIMANDS] are meant to do" , "Humiliate" , "Belittle, degrade" .)