First-grade mountain, hard ascent at first (5)
I believe the answer is:
alpha
'first-grade' is the definition.
(alpha can mean top class)
'mountain hard ascent at first' is the wordplay.
'mountain' becomes 'alp' (the Alps are a mountain range).
'at first' says to take the initial letters.
The first letters of 'hard ascent' is 'ha'.
'alp'+'ha'='ALPHA'
(Other definitions for alpha that I've seen before include "First (class)" , "leader in Athens, originally" , "Top grade" , "starter" , "Burrowing American rodent" .)