Cool approach to climbing mountain (5)
I believe the answer is:
chill
'cool' is the definition.
('chill' can be a synonym of 'cool')
'approach to climbing mountain' is the wordplay.
'approach to' says to take the initial letters.
'mountain' becomes 'hill' (I have seen 'Miniature mountain' mean 'hill' so perhaps 'mountain' could also mean 'hill').
The first letter of 'climbing' is 'c'.
'c'+'hill'='CHILL'
(Other definitions for chill that I've seen before include "Slight cold" , "Preserve by cold" , "Refrigerate" , "Make cooler" , "Make cool" .)