Horseman capturing a pillager (6)
I believe the answer is:
raider
'pillager' is the definition.
(I know that pillager can be written as raider)
'horseman capturing a' is the wordplay.
'horseman' becomes 'rider' (someone riding a horse).
'capturing' means one lot of letters goes inside another.
'rider' going around 'a' is 'RAIDER'.
(Other definitions for raider that I've seen before include "person taking food from larder" , "Depredator" , "One who makes surprise attack, perhaps to steal" , "looter" , "Surprise attacker" .)