Good person, impolite, left dessert (7)
I believe the answer is:
strudel
'dessert' is the definition.
(I've seen this before)
'good person impolite left' is the wordplay.
'good person' becomes 'st' (saint).
'impolite' becomes 'rude' (similar in meaning).
'left' becomes 'l' (common abbreviation).
'st'+'rude'+'l'='STRUDEL'
(Other definitions for strudel that I've seen before include "Fruit-filled pastry from Austria" , "Baked dish" , "food" , "German pastry dish" , "See 6" .)