Horse active in elevated region (8)
I believe the answer is:
mountain
'elevated region' is the definition.
'horse active in' is the wordplay.
'horse' becomes 'mount' (mount is a kind of horse).
'active' becomes 'a' (grammatical abbreviation).
'mount'+'a'+'in'='MOUNTAIN'
(Other definitions for mountain that I've seen before include "One of a range" , "High peak" , "Something made out of a molehill?" , "Eminence" , "may be in range?" .)