Open tart (4)
I believe the answer is:
'open tart' is the definition.
(a flan is an open-topped pastry)
This is all the clue.
(Other definitions for flan that I've seen before include "it may get jammed" , "Open tart with a sweet or savoury filling" , "pudding!" , "Open pastry or sponge case with tasty filling" , "Open pastry dish" .)