Attacker offing one horseman (5)
I believe the answer is:
rider
'horseman' is the definition.
(someone riding a horse)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for rider that I've seen before include "Condition added to a legislative bill" , "Horseman or add-on verdict by jury" , "Condition added to something already agreed" , "Horseman or supplementary jury verdict" , "Comment from jury" .)