Monsters devouring old mounted soldiers (8)
I believe the answer is:
dragoons
'soldiers' is the definition.
(dragoon is a kind of soldier)
'monsters devouring old mounted' is the wordplay.
I cannot quite understand how this works, but
'old' could be 'o' (abbreviation) and 'o' is present in the answer.
'monsters' could be 'dragons' (dragon is a kind of monster) and 'dragons' is found in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for dragoons that I've seen before include "Compels by force; soldier" , "Forces" , "Bullies" , "Heavily armed cavalrymen" .)