Like Granny Smiths (4)
I believe the answer is:
tart
'like granny smiths' 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 "Sour-tasting pie?" , "Sharp or sour in taste" , "Cutting, pastry" , "Sour -- acid" , "Acidulous" .)