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 really understand how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is found in the answer.
'monsters' could be 'dragons' (dragon is a kind of monster) and 'dragons' is found in the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
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" .)