Transgress for a bad purpose (6)
I believe the answer is:
offend
'transgress' is the definition.
(I've seen this in another clue)
'bad purpose' is the wordplay.
'bad' becomes 'off' (eg meat that's gone off is bad).
'purpose' becomes 'end' (both can mean a goal).
'off'+'end'='OFFEND'
'for a' is the link.
(Other definitions for offend that I've seen before include "Cause displeasure to, insult" , "Be disagreeable to" , "Insult - sin" , "Annoy - upset" , "Distress" .)