Duty for this French military group (4,5)
I believe the answer is:
task force
'military group' is the definition.
(I've seen this before)
'duty for this french' is the wordplay.
'duty' becomes 'task' (task is a kind of duty).
'this french' becomes 'ce' (I've seen this in other clues).
'task'+'for'+'ce'='TASK-FORCE'
(Other definitions for task force that I've seen before include "Squad for undertaking" , "specially organised unit" , "Group specially organised for particular operation" , "Temporary unit to accomplish an objective" , "Temporary group to do a job" .)