Darren's off on a mission (6)
I believe the answer is:
errand
'mission' is the definition.
(I've seen this before)
'darren's off' is the wordplay.
'off' indicates anagramming the letters.
'darren' with letters rearranged gives 'ERRAND'.
'on a' is the link.
(Other definitions for errand that I've seen before include "An ...... of mercy is a journey to relieve distress" , "Mission usually for others" , "Mission (usually run!)" , "run one" , "light task" .)