Attacker booting out a cavalryman? (5)
I believe the answer is:
rider
'cavalryman?' is the definition.
(thesaurus)
'attacker booting out a' is the wordplay.
'attacker' becomes 'raider' (I've seen this before).
'booting out' is a deletion indicator.
'raider' with 'a' taken out is 'RIDER'.
(Other definitions for rider that I've seen before include "Equestrian added by a jury?" , "Equestrian addition by a jury" , "Motorcyclist" , "Contractual clause" , "in the dressing room before the show?" .)