Darren sent off on mission (6)
I believe the answer is:
errand
'mission' is the definition.
(I've seen this before)
'darren sent off' is the wordplay.
'sent off' indicates anagramming the letters.
'darren' is an anagram of 'ERRAND'.
'on' is the link.
(Other definitions for errand that I've seen before include "In purposeful trip" , "Short assignment" , "light task" , "Short mission -- ran red (anag)" , "Short trip to perform required task" .)