Fiery monster devouring old soldier (7)
I believe the answer is:
dragoon
'soldier' is the definition.
(I know that dragoon is a type of cavalryman)
'fiery monster devouring old' is the wordplay.
'fiery monster' becomes 'dragon' (I've seen this before).
'devouring' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'dragon' going around 'o' is 'DRAGOON'.
(Other definitions for dragoon that I've seen before include "Browbeat" , "Constrain" , "Harass" , "Cavalryman; coerce (into doing something)" , "Shanghai" .)