Bishop about to have one dairy product (4)
I believe the answer is:
brie
'dairy product' is the definition.
(I know that Brie is a cheese)
'bishop about to have one' is the wordplay.
'bishop' becomes 'B' (abbreviation for bishop in chess).
'about' becomes 're' ('re' means regarding or about).
'to have' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
're' going around 'i' is 'rie'.
'b'+'rie'='BRIE'
(Other definitions for brie that I've seen before include "Bergamot-flavoured tea" , "Soft and creamy mild white cheese" , "Variety of cheese" , "Food" , "dairy product" .)