Stranger supporting unknown number held in mountain (9)
I believe the answer is:
foreigner
'stranger' is the definition.
(foreigner is a kind of stranger)
'supporting unknown number held in mountain' is the wordplay.
'supporting' becomes 'for'.
'unknown number' becomes 'n'.
'held in' indicates putting letters inside.
'mountain' becomes 'eiger' (Swiss mountain).
'n' placed into 'eiger' is 'eigner'.
'for'+'eigner'='FOREIGNER'
(Other definitions for foreigner that I've seen before include "chauvinist holds in contempt" , "One from abroad" , "asylum seeker?" , "Stranger, outsider" , "Alien" .)