Military group (4)
'military group' is the definition.
(group of soldiers)
This is all the clue.
(Other definitions for unit that I've seen before include "detachment maybe" , "Small military detachment" , "Military team" , "Detachment of soldiers" , "One; military group" .)
(Other definitions for army that I've seen before include "Military unit" , "Military land forces" , "fighters" , "A great number (of soldiers?)" , "service" .)