Article dropped by one attacking cavalryman (5)
I believe the answer is:
rider
'cavalryman' is the definition.
(thesaurus)
'article dropped by one attacking' is the wordplay.
'article' becomes 'a' (indefinite article).
'dropped by' is a deletion indicator.
'one attacking' becomes 'raider' (a person participating in an attack).
'raider' with 'a' taken out is 'RIDER'.
(Other definitions for rider that I've seen before include "Comment from jury" , "extra weight needed for balance?" , "One backing a horse" , "One in the saddle" , "Equestrian added by a jury?" .)