Unusual way to the mountains (7)
I believe the answer is:
strange
'unusual' is the definition.
(I know that unusual can be written as strange)
'way to the mountains' is the wordplay.
'way' becomes 'st' (abbreviation for street).
'to the mountains' becomes 'range' (range of mountains).
'st'+'range'='STRANGE'
(Other definitions for strange that I've seen before include "Odd, alien" , "Odd, not familiar" , "Offbeat" , "- - Fruit (song)" , "Extraordinary" .)