Cold in head of boy going fast (7)
I believe the answer is:
bracing
'cold' is the definition.
(bracing can describe cold weather)
'head of boy going fast' is the wordplay.
'head of' says to take the initial letters.
'going fast' becomes 'racing' (to race is to go fast).
The first letter of 'boy' is 'b'.
'b'+'racing'='BRACING'
'in' is the link.
(Other definitions for bracing that I've seen before include "Imparting vitality" , "Supporting" , "Refreshing" , "Fresh and invigorating" , "Cold" .)