Largely hair-raising mountain route? (3)
I believe the answer is:
col
'mountain route?' is the definition.
(a col is a mountain pass)
'largely hair-raising' is the wordplay.
'largely' means to remove the last letter (most but not all of the letters).
'hair' becomes 'lock' (a lock of hair).
'raising' says the letters should be written backwards (in a down clue, letters go up).
'lock' with its final letter removed is 'loc'.
'loc' written backwards gives 'COL'.
(Other definitions for col that I've seen before include "Low area between peaks" , "A pass through ridge" , "route through mountains" , "Depression among ecologists" , "High mountain pass" .)