Mostly chilly passage through mountains (3)
I believe the answer is:
col
'passage through mountains' is the definition.
(a col is a mountain pass)
'mostly chilly' is the wordplay.
'mostly' means to remove the last letter.
'chilly' becomes 'cold' (I've seen this before).
'cold' with its last letter removed is 'COL'.
(Other definitions for col that I've seen before include "Mountain saddle" , "Young horse almost" , "High mountain pass" , "Mountain defile" , "dip?" .)