Beginning or ending of mountain climb (7)
I believe the answer is:
nascent
'beginning' is the definition.
(something nascent is just beginning or starting)
'ending of mountain climb' is the wordplay.
'ending of' indicates one should take the final letters.
'climb' becomes 'ascent' (synonyms).
The last letter of 'mountain' is 'n'.
'n'+'ascent'='NASCENT'
'or' is the link.
(Other definitions for nascent that I've seen before include "just coming into effect" , "Starting to grow" , "Beginning to show signs of potential" , "starting to mature" , "Incipient" .)