One with a will? Try a mountain! (8)
I believe the answer is:
testator
'one with a will?' is the definition.
(someone who has made a will)
'try a mountain' is the wordplay.
'try' becomes 'test' (synonyms).
'mountain' becomes 'tor'.
'test'+'a'+'tor'='TESTATOR'
(Other definitions for testator that I've seen before include "One making a will" , "One leaving" , "Will provider" , "Person who has made a will" , "Man who makes a will" .)