Cheese firm turning hard inside bread (7)
I believe the answer is:
brioche
'bread' is the definition.
(brioche is a kind of bread)
'cheese firm turning hard inside' is the wordplay.
'cheese' becomes 'brie' (Brie is an example).
'firm' becomes 'co' (abbreviation for company).
'turning' says the letters should be written backwards.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'inside' means one lot of letters goes inside another.
'co' written backwards gives 'oc'.
'oc'+'h'='och'
'brie' placed around 'och' is 'BRIOCHE'.
(Other definitions for brioche that I've seen before include "Sweet yeast bread" , "Hire cob (anag) - bread" , "Sponge cake or roll" , "Soft roll sometimes mixed with currants" , "French bread roll" .)