Sour pie (4)
I believe the answer is:
tart
This clue is a double definition.
'sour' is the first definition.
(similar in meaning)
'pie' is the second definition.
(I know that tart is a type of pie)
(Other definitions for tart that I've seen before include "Bitter - sweet - floozie" , "Sour-tasting pie?" , "Cutting, pastry" , "Biting; flan" , "Bitter - pudding" .)