Darren went off on a 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' is an anagram indicator.
'darren' anagrammed gives 'ERRAND'.
'on a' is the link.
(Other definitions for errand that I've seen before include "Message run" , "Short commission" , "Short journey done for others" , "Short mission -- ran red (anag)" , "Task, message" .)