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' says the letters should be written in reverse.
'in' means one lot of letters goes inside another.
'sub' in reverse letter order is 'bus'.
'bus' inserted into 'ae' is 'ABUSE'.
(Other definitions for abuse that I've seen before include "Misemploy - invective" , "Violate" , "Mistreatment" , "Inveigh against" , "Cruel treatment" .)