Confederates (5)
'confederates' is the definition.
('ally' can be a synonym of 'confederate')
This is the entire clue.
(Other definitions for allies that I've seen before include "Co-operating states" , "Countries working together" , "Colleagues" , "Partners or confederates" , "Joins forces" .)
(Other definitions for posse that I've seen before include "Group of friends together" , "Law-enforcers" , "Troop at the service of a local sheriff" , "legal team" , "Sheriff's band of men" .)