Transgress with mission (6)
I believe the answer is:
errand
'mission' is the definition.
(I've seen this in another clue)
'transgress with' is the wordplay.
'transgress' becomes 'err' (I've seen this before).
'with' becomes 'and'.
'err'+'and'='ERRAND'
(Other definitions for errand that I've seen before include "Short journey to carry message for Renard" , "Short journey on behalf of another" , "Journey to deliver message etc" , "Short journey to get or bring" , "Messenger's job" .)