Army sergeant major's containing hostilities (5)
I believe the answer is:
swarm
'army' is the definition.
(I've seen this before)
'sergeant major's containing hostilities' is the wordplay.
'sergeant major' becomes 'SM' (abbreviation for sergeant major).
'containing' means one lot of letters goes inside another.
'hostilities' becomes 'war' (hostilities between two nations).
'sm' placed around 'war' is 'SWARM'.
(Other definitions for swarm that I've seen before include "Horde" , "Arrive in numbers" , "Group of flies" , "Throng of insects" , "Large group of bees" .)