Just beginning last of mountain climb (7)
I believe the answer is:
nascent
'just beginning' is the definition.
(I've seen this before)
'last of mountain climb' is the wordplay.
'last of' says to take the final letters.
'climb' becomes 'ascent' ('ascent' can be a synonym of 'climb').
The last letter of 'mountain' is 'n'.
'n'+'ascent'='NASCENT'
(Other definitions for nascent that I've seen before include "Starting to develop" , "Emerging, in the process of being born" , "Newly-formed" , "Starting to grow" , "starting to mature" .)