Fast, but cold (5)
I believe the answer is:
nippy
This might be a double definition.
'fast' is the first definition.
Both the answer and definition are adjectives. Perhaps you can see a link between them that I can't see?
'cold' is the second definition.
(similar in meaning)
'but' acts as a link.
Can you help me to learn more?
(Other definitions for nippy that I've seen before include "Slightly cold?" , "Fast - cold" , "progressing rapidly" , "Cold - nimble" , "Nimble; chilly" .)