Darren went off on mission (6)
I believe the answer is:
errand
'mission' is the definition.
(I've seen this before)
'darren went off' is the wordplay.
'went off' indicates an anagram.
'darren' with letters rearranged gives 'ERRAND'.
'on' is the link.
(Other definitions for errand that I've seen before include "A little job" , "Commission, task" , "Journey made on someone else's behalf" , "Short journey to get or bring" , "Mission (usually run!)" .)