Cavalry arrests American attackers (7)
I believe the answer is:
raiders
'attackers' is the definition.
'cavalry arrests american' is the wordplay.
'cavalry' becomes 'riders'.
'arrests' means one lot of letters goes inside another (inserted letters are contained or arrested).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'riders' enclosing 'a' is 'RAIDERS'.
(Other definitions for raiders that I've seen before include "Marauders" , "' - - of the Lost Ark'" , "Marauding attackers" , "Attacking force" .)