For one mounted, running battle's a possibility (8)
I believe the answer is:
gettable
'a possibility' is the definition.
(something it's possible to get)
'for one mounted running battle's' is the wordplay.
'for one' becomes 'eg' (I've seen this in other clues).
'mounted' shows that the letters should be reversed in order.
'running' indicates anagramming the letters.
'eg' written backwards gives 'ge'.
'battle' anagrammed gives 'ttable'.
'ge'+'ttable'='GETTABLE'
(Other definitions for gettable that I've seen before include "in range" , "Can be achieved" , "Not out of reach" .)