Humble sailor, like slave, ultimately (5)
I believe the answer is:
abase
'humble' is the definition.
('abase' can be a synonym of 'humble')
'sailor like slave ultimately' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'like' becomes 'as'.
'ultimately' says to take the final letters.
The last letter of 'slave' is 'e'.
'ab'+'as'+'e'='ABASE'
(Other definitions for abase that I've seen before include "Lower" , "Degrade (oneself)" , "Cast down" , "what 11 [REPRIMANDS] are meant to do" , "Belittle, degrade" .)