Horseman catching a marauder (6)
I believe the answer is:
raider
'marauder' is the definition.
(I've seen this before)
'horseman catching a' is the wordplay.
'horseman' becomes 'rider' (someone riding a horse).
'catching' indicates putting letters inside.
'rider' enclosing 'a' is 'RAIDER'.
(Other definitions for raider that I've seen before include "Marauder" , "Armed insurgent" , "One attacks" , "Surprise attacker" , "person who's involved in sacking?" .)