Mistreats us in base fashion (6)
I believe the answer is:
abuses
'mistreats' is the definition.
('abuse' can be a synonym of 'mistreat')
'us in base fashion' is the wordplay.
'in' is an insertion indicator.
'fashion' indicates anagramming the letters.
'base' is an anagram of 'abes'.
'us' inserted within 'abes' is 'ABUSES'.
(Other definitions for abuses that I've seen before include "Treats cruelly" , "Curses" , "Employs or deals with incorrectly" , "Reviles and insults" , "Maltreats people or things" .)