Dessert left in cooler (4)
I believe the answer is:
flan
'dessert' is the definition.
(flan is a kind of dessert)
'left in cooler' is the wordplay.
'left' becomes 'L' (common abbreviation).
'in' indicates putting letters inside.
'cooler' becomes 'fan' (something which cools).
'l' inserted inside 'fan' is 'FLAN'.
(Other definitions for flan that I've seen before include "Baked and filled pastry case dish" , "Quiche" , "Sweet/savoury open tart" , "Open pastry dessert" , "Open-topped pastry dish" .)