Pastry item (3)
(Other definitions for pie that I've seen before include "Slapstick missile" , "Pastry product" , "In the sky, it's an empty promise" , "In the sky, it's a hopeless idea" , "In the sky, this dish is impractical" .)
'pastry item' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tart that I've seen before include "Somewhat sour like flat cake" , "sour type of pie" , "Sharp tasting- like kind of pie?" , "Sharp, acidic" , "See 10" .)