Mission to go wrong, in addition (6)
I believe the answer is:
errand
'mission' is the definition.
(I've seen this before)
'go wrong in addition' is the wordplay.
'go wrong' becomes 'err' (I've seen this before).
'in addition' becomes 'and' (I've seen this before).
'err'+'and'='ERRAND'
'to' is the link.
(Other definitions for errand that I've seen before include "Short journey to get or bring" , "Short trip for a message" , "An ...... of mercy is a journey to relieve distress" , "Short journey to get or carry message" , "Chore" .)