Infantry decapitated by bowmen (7)
I believe the answer is:
archers
'bowmen' is the definition.
('archer' can be a synonym of 'bowman')
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for archers that I've seen before include "Bowmen on BBC radio?" , "old soldiers?" , "Their sport is the bow and arrow" , "Part of army once" , "Toxophilites" .)