Distance set to change over mountains (8)
I believe the answer is:
estrange
'distance' is the definition.
(both can mean to create distance or separation)
'set to change over mountains' is the wordplay.
'to change' indicates an anagram.
'over' means one lot of letters go next to another.
'mountains' becomes 'range' (range of mountains).
'set' anagrammed gives 'est'.
'est'+'range'='ESTRANGE'
(Other definitions for estrange that I've seen before include "Ten gears (anag.)" , "Forfeit friendship of, make hostile" , "Make enemy of sergeant" , "Make bad friends with sergeant" , "Lose friendship" .)