Invader taking out a horseman (5)
I believe the answer is:
rider
'horseman' is the definition.
(someone riding a horse)
'invader taking out a' is the wordplay.
'invader' becomes 'raider' (I've seen this before).
'taking out' is a deletion indicator.
'raider' with 'a' taken out is 'RIDER'.
(Other definitions for rider that I've seen before include "Horseman or addendum" , "Addition to verdict" , "Added proviso" , "Motorcyclist" , "Jury's added comment" .)