Sick accustomed to being mistreated (3-4)
I believe the answer is:
ill-used
'being mistreated' is the definition.
(I've seen this in another clue)
'sick accustomed' is the wordplay.
'sick' becomes 'ill' (similar in meaning).
'accustomed' becomes 'used' (I've seen this in another clue).
'ill'+'used'='ILL-USED'
'to' is the link.
(Other definitions for ill-used that I've seen before include "Mistreated" , "Badly treated, exploited" , "Maltreated" .)