Mission to go wrong and end (6)
I believe the answer is:
errand
'mission' is the definition.
(I've seen this before)
'go wrong and end' is the wordplay.
'go wrong' becomes 'err' (I've seen this in another clue).
'and' means one lot of letters go next to another.
'end' becomes 'and' (I can't justify this - if you can you should believe this answer much more).
'err'+'and'='ERRAND'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for errand that I've seen before include "Mission (usually run!)" , "A little job" , "Useful journey" , "Trip for small task" , "Charge - mission" .)