Sudden cool-down (4,4)
I believe the answer is:
cold snap
'sudden cool-down' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cold snap that I've seen before include "Short wintry spell" , "Sudden brief spell of chilly weather" , "Sudden chilly period" , "Brief spell of freezing weather" , "Brief chilly period" .)