Small sour fruit used for making jam and jelly (4,5)
I believe the answer is:
crab apple
(Other definitions for crab apple that I've seen before include "Sour fruit used for jam or jelly" , "Small sour version of common orchard fruit" , "unappetizing fruit" , "A small sour fruit - will it keep the doctor away?" , "A small sour fruit, part crustacean" .)