Fighter, being mountain-based, occupies lower ground (8)
I believe the answer is:
toreador
'fighter' is the definition.
(toreador is a kind of fighter)
'mountain-based occupies lower ground' is the wordplay.
I cannot really understand how this works, but
'mountain' could be 'oread' (I've seen this in another clue) and 'oread' is present in the answer.
The remaining letters 'tor' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'being' acts as a link.
Can you help me to learn more?
(Other definitions for toreador that I've seen before include "one features in The Ring" , "One who fights bulls" , "Mounted bullfighter" , "A bullfighter, especially on horseback" , "Man honoured to be given an ear" .)