Dessert restaurant served up (4)
I believe the answer is:
tart
'dessert' is the definition.
(type of dessert)
'restaurant served up' is the wordplay.
'restaurant' becomes 'trat' (short for trattoria).
'served up' shows that the letters should be reversed in order (in a down clue, going upwards).
'trat' in reverse letter order is 'TART'.
(Other definitions for tart that I've seen before include "Acid-tasting; pastry" , "Filled pastry dish" , "Sour-tasting pie?" , "Sour -- acid" , "loose woman" .)