Fruit roll filling poor, loose crusts (4,5)
I believe the answer is:
crab apple
'fruit' is the definition.
(crab apple is a kind of fruit)
'roll filling poor loose crusts' is the wordplay.
I cannot quite see how this works, but
'roll' could be 'bap' (bap can mean a bread roll) and 'bap' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for crab apple that I've seen before include "Fruit tree with sour bright red crop" , "unappetizing fruit" , "Tree bearing fruit for jelly" , "Wild fruit used for jam" , "A small sour fruit - will it keep the doctor away?" .)