Humiliates sailor on rough seas (6)
I believe the answer is:
abases
'humiliates' is the definition.
('abase' can be a synonym of 'humiliate')
'sailor on rough seas' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'on' indicates putting letters inside.
'rough' indicates an anagram.
'seas' anagrammed gives 'ases'.
'ab' placed into 'ases' is 'ABASES'.
(Other definitions for abases that I've seen before include "degrades" , "humbles" , "Takes down" , "Humiliates, belittles" .)