Cool down (5)
I believe the answer is:
chill
'cool down' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for chill that I've seen before include "Refrigerate" , "A feeling of coldness" , "Unpleasent coldness in the air" , "Preserve by cold" , "Slang injunction to relax - it's cold" .)