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 judge whether this definition defines the answer.
'be about 40%' is the wordplay.
'about' means one lot of letters goes inside another.
'40%' becomes 'ri' (I am not sure about this - if you are sure you should believe this answer much more).
'be' enclosing '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" .)