Transgress for a bad purpose (6)
I believe the answer is:
offend
'transgress' is the definition.
(I know that transgress can be written as offend)
'a bad purpose' is the wordplay.
'a bad' becomes 'off' (I've seen this before).
'purpose' becomes 'end' (I've seen this before).
'off'+'end'='OFFEND'
'for' acts as a link.
(Other definitions for offend that I've seen before include "Cause displeasure to, insult" , "Be disagreeable to" , "Insult - sin" , "Annoy - upset" , "Distress" .)