Climb a trail (6)
I believe the answer is:
ascent
'climb' is the definition.
('ascent' can be a synonym of 'climb')
'a trail' is the wordplay.
'trail' becomes 'scent' (I've seen this before).
'a'+'scent'='ASCENT'
(Other definitions for ascent that I've seen before include "way up the mountain" , "Going up" , "A rise in status; climb" , "Hill" , "Upward journey" .)