Old volunteer force (8)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
yeomanry
(Other definitions for yeomanry that I've seen before include "Former volunteer cavalry force, many yore (anag.)" , "Group of bodyguards" , "cavalrymen once" , "Group of small estate owners (historical)" , "Class of small freeholders cultivating their own land" .)