Rich crumbly biscuit (10)
I believe the answer is:
'rich crumbly biscuit' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for shortbread that I've seen before include "Biscuit with plenty of butter" , "sweet but easily crumbling" , "Crumbly biscuit rich in butter" , "biscuits" , "Brittle buttery cake" .)