Start of alpine trail an upward slope (6)
I believe the answer is:
ascent
'an upward slope' is the definition.
(I've seen this in another clue)
'start of alpine trail' is the wordplay.
'start of' indicates taking the first letters.
'trail' becomes 'scent' (an animal's trail is its scent).
The initial letter of 'alpine' is 'a'.
'a'+'scent'='ASCENT'
(Other definitions for ascent that I've seen before include "Upward journey" , "Mountain path" , "Hill" , "Incline" , "Going up" .)