Budding mountaineer's fourth climb (7)
I believe the answer is:
nascent
'budding' is the definition.
(I've seen this before)
'mountaineer's fourth climb' is the wordplay.
'mountaineer's fourth' becomes 'n' (4th letter of 'mountaineer').
'climb' becomes 'ascent' (synonyms).
'n'+'ascent'='NASCENT'
(Other definitions for nascent that I've seen before include "not yet mature" , "Dawning" , "coming into circulation?" , "Beginning to develop, embryonic" , "Promising potential" .)