Someone pillaging downs a horseman (5)
I believe the answer is:
rider
'horseman' is the definition.
(I've seen this before)
'someone pillaging downs a' is the wordplay.
'someone pillaging' becomes 'raider'.
'downs' indicates named letters should be taken away (down can mean to stop using or remove from service).
'raider' with 'a' taken out is 'RIDER'.
(Other definitions for rider that I've seen before include "recommendation made in court" , "Horseman or cyclist" , "Person on horse" , "supported by an Arab, perhaps" , "Jury's added comment" .)