Crossword Genius

Darren went off on a mission (6)

Ross

I believe the answer is:

errand

Here is my best explanation:

'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.