Maltreat stand-in brought round in AE (5)
I believe the answer is:
abuse
'maltreat' is the definition.
(synonyms)
'stand-in brought round in ae' is the wordplay.
'stand in' becomes 'sub' (synonyms).
'brought round' is a reversal indicator.
'in' means one lot of letters goes inside another.
'sub' backwards is 'bus'.
'bus' placed inside 'ae' is 'ABUSE'.
(Other definitions for abuse that I've seen before include "Misemploy - invective" , "Violate" , "Mistreatment" , "Inveigh against" , "Cruel treatment" .)