Monster devouring old mounted soldier (7)
I believe the answer is:
dragoon
'mounted soldier' is the definition.
(I've seen this before)
'monster devouring old' is the wordplay.
'monster' becomes 'dragon' (dragon is a kind of monster).
'devouring' is an insertion indicator.
'old' becomes 'o' (abbreviation).
'dragon' placed around 'o' is 'DRAGOON'.
(Other definitions for dragoon that I've seen before include "Cavalry officer" , "Cavalier" , "Coerce; mounted soldier" , "Former heavily-armed cavalryman" , "Former heavily armed cavalryman" .)