Pillagers -- those on horseback circling area (7)
I believe the answer is:
raiders
'pillagers' is the definition.
('raider' can be a synonym of 'pillager')
'those on horseback circling area' is the wordplay.
'those on horseback' becomes 'riders' (horse riders).
'circling' is an insertion indicator (circling can mean going around or containing).
'area' becomes 'a' (maths abbreviation).
'riders' placed around 'a' is 'RAIDERS'.
(Other definitions for raiders that I've seen before include "Marauding attackers" , "Marauders" , "' - - of the Lost Ark'" , "Attacking force" .)