Gordon Riots, involving a mounted infantryman (7)
I believe the answer is:
dragoon
'mounted infantryman' is the definition.
(soldier on horseback)
'gordon riots involving a' is the wordplay.
'riots' indicates an anagram.
'involving' indicates putting letters inside.
'gordon' with letters rearranged gives 'drgoon'.
'drgoon' enclosing 'a' is 'DRAGOON'.
(Other definitions for dragoon that I've seen before include "Former heavily-armed cavalryman" , "Constrain" , "bring into line" , "Cavalry soldier" , "Former heavily armed cavalryman" .)