Sailor accustomed to being ill-treated (6)
I believe the answer is:
abused
'being ill-treated' is the definition.
('abused' can be a synonym of 'ill-treated')
'sailor accustomed' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'accustomed' becomes 'used' (I've seen this before).
'ab'+'used'='ABUSED'
'to' is the link.
(Other definitions for abused that I've seen before include "Reviled, or used wrongly like drugs" , "Offended" , "Took undue advantage of" , "Criminally maltreated" , "Insulted; violated" .)