Equestrian rounds a marauder (6)
I believe the answer is:
raider
'marauder' is the definition.
(I've seen this before)
'equestrian rounds a' is the wordplay.
'equestrian' becomes 'rider' (I've seen this before).
'rounds' means one lot of letters goes inside another.
'rider' enclosing 'a' is 'RAIDER'.
(Other definitions for raider that I've seen before include "Maker of surprise attack" , "One plundering" , "One who makes surprise attack, perhaps to steal" , "looter" , "Pillager, plunderer" .)