Left in the cooler it may be jammed (4)
I believe the answer is:
flan
'it may be jammed' is the definition.
The definition suggests a singular noun which matches the answer.
'left in the cooler' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' indicates putting letters inside.
'the cooler' becomes 'fan' (something which cools).
'l' going into 'fan' is 'FLAN'.
Can you help me to learn more?
(Other definitions for flan that I've seen before include "Baked and filled pastry case dish" , "Quiche" , "Open pastry filled with savouries, fruit or custard" , "Flat open tart" , "Open-topped tart" .)