Darren is off on a mission (6)
I believe the answer is:
errand
'mission' is the definition.
(thesaurus)
'darren is off' is the wordplay.
'is off' indicates anagramming the letters.
'darren' with letters rearranged gives 'ERRAND'.
'on a' acts as a link.
(Other definitions for errand that I've seen before include "Chore" , "Native of a former German kingdom" , "A short journey to deliver orget goods" , "Delivery/collection trip" , "Short journey to get or carry message" .)