Pastry dish left in cooler (4)
I believe the answer is:
flan
'pastry dish' is the definition.
(I've seen this before)
'left in cooler' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' indicates putting letters inside.
'cooler' becomes 'fan' (something which cools).
'l' put into 'fan' is 'FLAN'.
(Other definitions for flan that I've seen before include "Baked and filled pastry case dish" , "Quiche-like dish" , "Open pastry dessert" , "Open pastry case with savoury filling" , "Open-topped tart" .)