Military group occupying battlement ask for ceasefire (4,5)
I believe the answer is:
task force
'military group' is the definition.
(military group with a particular mission)
'occupying battlement ask for ceasefire' is the wordplay.
'occupying' says the answer is hidden in the clue (letters for the answer occupy part of the clue).
'TASK-FORCE' is hidden within 'battlement ask for ceasefire'.
(Other definitions for task force that I've seen before include "Group for special operation" , "working party" , "special unit" , "subcommittee" , "Temporary unit to accomplish an objective" .)