Going up a trail (6)
I believe the answer is:
ascent
'going up' is the definition.
(ascending means going up)
'a trail' is the wordplay.
'trail' becomes 'scent' (an animal's trail is its scent).
'a'+'scent'='ASCENT'
(Other definitions for ascent that I've seen before include "made by Hilary and Tenzing" , "Alpine expedition" , "Act of moving upwards" , "Upward slope" , "Maybe promotion" .)