Soldiers Mary treated (4)
I believe the answer is:
army
'soldiers' is the definition.
(group of soldiers)
'mary treated' is the wordplay.
'treated' indicates anagramming the letters.
'mary' anagrammed gives 'ARMY'.
(Other definitions for army that I've seen before include "Military land forces" , "Horde" , "See 7" , "Military land force" , "A defence force" .)