Cheese that’s firm turning hard inside bread (7)
I believe the answer is:
brioche
'bread' is the definition.
(brioche is a kind of bread)
'cheese that's firm turning hard inside' is the wordplay.
'cheese' becomes 'brie' (Brie is an example).
'that's firm' becomes 'co' (a firm is a company).
'turning' says the letters should be written in reverse.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'inside' means one lot of letters goes inside another.
'co' reversed gives 'oc'.
'oc'+'h'='och'
'brie' going around 'och' is 'BRIOCHE'.
(Other definitions for brioche that I've seen before include "French bread roll" , "Idol" , "Small French roll" , "Light sweetish roll" , "Sponge cake or roll" .)