Go wrong with mission (6)
I believe the answer is:
errand
'mission' is the definition.
(I've seen this before)
'go wrong with' is the wordplay.
'go wrong' becomes 'err' (I've seen this in another clue).
'with' becomes 'and'.
'err'+'and'='ERRAND'
(Other definitions for errand that I've seen before include "Job usually involving a short trip" , "Message run" , "Short trip to perform required task" , "Messenger's job" , "Mission usually for others" .)