Do wrong for a bad purpose (6)
I believe the answer is:
offend
'do wrong' is the definition.
(thesaurus)
'a bad purpose' is the wordplay.
'a bad' becomes 'off' (eg meat that's gone off is bad).
'purpose' becomes 'end' (I've seen this before).
'off'+'end'='OFFEND'
'for' is the link.
(Other definitions for offend that I've seen before include "Violate the law" , "Outrage" , "Be disagreeable to" , "tread on someone's toes?" , "Act in disregard of the law" .)