Climb, avoiding first trail (5)
I believe the answer is:
scent
'trail' is the definition.
(an animal's trail is its scent)
'climb avoiding first' is the wordplay.
'climb' becomes 'ascent' (synonyms).
'avoiding' is a deletion indicator.
'first' becomes 'a'.
'ascent' with 'a' taken away is 'SCENT'.
(Other definitions for scent that I've seen before include "Perfume for the tracker" , "Fragrance" , "Trail" , "Clue" , "Pleasant smell" .)