French cheese stores have rich cake (7)
I believe the answer is:
brownie
'rich cake' is the definition.
'brownie' can be an answer for 'cake' (brownie is a kind of cake). I'm not sure about the 'rich' bit.
'french cheese stores have' is the wordplay.
'french cheese' becomes 'brie' (brie is a French cheese).
'stores' means one lot of letters goes inside another.
'have' becomes 'own' (to have something is to own it).
'brie' enclosing 'own' is 'BROWNIE'.
Can you help me to learn more?
(Other definitions for brownie that I've seen before include "Girl in uniform" , "sweet thing" , "Benevolent helper" , "Junior Girl Scout" , "helpful girl?" .)