The first to demand a mission (4)
I believe the answer is:
task
'a mission' is the definition.
(I've seen this in another clue)
'the first to demand' is the wordplay.
'the first to' becomes 't' (1st letter of 'to').
'demand' becomes 'ask' (synonyms).
't'+'ask'='TASK'
(Other definitions for task that I've seen before include "Set piece of work" , "Burden with work" , "Job of work" , "exercise" , "Allocate work" .)