They attack people on horseback surrounding area (7)
I believe the answer is:
raiders
'they attack' is the definition.
(people doing raids)
'people on horseback surrounding area' is the wordplay.
'people on horseback' becomes 'riders' (horse riders).
'surrounding' indicates putting letters inside.
'area' becomes 'a' (maths abbreviation).
'riders' placed around 'a' is 'RAIDERS'.
(Other definitions for raiders that I've seen before include "' - - of the Lost Ark'" , "Attacking force" , "Marauders" , "Marauding attackers" .)