Tart left in cooler (4)
I believe the answer is:
flan
'tart' 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' (I've seen this in another clue).
'l' put within 'fan' is 'FLAN'.
(Other definitions for flan that I've seen before include "Open pastry case with savoury filling" , "Baked dish filled with fruit or custard" , "Open-topped pie" , "Quiche-like dish" , "Quiche" .)