Like Granny Smith apples (4)
I believe the answer is:
tart
'like granny smith apples' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tart that I've seen before include "Bitter - pudding" , "Sharp in manner, biting" , "Open pastry filled with apple or jam" , "loose woman" , "Tasting sour like a lemon" .)