Cooler day starts to disappear, gradually ending (6)
I believe the answer is:
fridge
'cooler' is the definition.
(I've seen this before)
'day starts to disappear gradually ending' is the wordplay.
'day' becomes 'fri'.
'starts to' suggests taking the first letters.
The first letters of 'disappear gradually ending' is 'dge'.
'fri'+'dge'='FRIDGE'
(Other definitions for fridge that I've seen before include "Food cooler" , "A domestic appliance" , "Cooler in kitchen" , "Cooling cabinet (abbr)" .)