Be about 40% right where cheese is concerned (4)
I believe the answer is:
brie
'where cheese is concerned' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'be about 40 right' is the wordplay.
'about' indicates putting letters inside.
'40 right' becomes 'ri' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'be' placed around 'ri' is 'BRIE'.
Can you help me to learn more?
(Other definitions for brie that I've seen before include "French produce" , "Food" , "French 8 across" , "cheese-producing region" , "Round soft Fr. cheese" .)