Hog's back, cooler when heading off (5)
I believe the answer is:
ridge
'hog's back' is the definition.
(I've seen this before)
'cooler when heading off' is the wordplay.
'cooler' becomes 'fridge' (I've seen this before).
'when heading off' means to remove the first letter.
'fridge' with its first letter taken away is 'RIDGE'.
(Other definitions for ridge that I've seen before include "Peak" , "Raised strip of land" , "Backbone of a whale" , "Highland area" , "Narrow raised strip as between hills" .)