Be about 40% right where cheese is concerned (4)
I believe the answer is:
brie
'right where cheese is concerned' is the definition.
I can't tell whether this definition defines the answer.
'be about 40%' is the wordplay.
'about' is an insertion indicator.
'40%' becomes 'ri' (I can't explain this - if you can you should give a lot more credence to this answer).
'be' going 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" .)