Volunteers risked having cover blown twice in assignment (4)
I believe the answer is:
task
'assignment' is the definition.
(I've seen this before)
'volunteers risked having cover blown twice' is the wordplay.
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'having cover blown twice' indicates the central letters (two layers of external letters are removed).
The middle of 'risked' is 'sk'.
'ta'+'sk'='TASK'
'in' acts as a link.
(Other definitions for task that I've seen before include "Difficult piece of work" , "exercise" , "Piece of work to be done" , "Sort of force" , "Assignment" .)