Private soldiers? Monsters (6)
I believe the answer is:
giants
'monsters' is the definition.
(type of monster or creature)
'private soldiers?' is the wordplay.
'private' becomes 'GI' (GI can informally mean a US soldier).
'soldiers?' becomes 'ants' (soldier is a type of ant).
'gi'+'ants'='GIANTS'
(Other definitions for giants that I've seen before include "Huge legendary beings" , "Large mythical beings" , "Behbmoths, titans" , "they're big" , "Behemoths, titans" .)