What's served up in their breakfast? Cheese (4)
I believe the answer is:
brie
'cheese' is the definition.
(I've seen this before)
'what's served up in their breakfast?' is the wordplay.
'what's served up' says the letters should be written in reverse.
'in' indicates a hidden word.
'eirb' can be found hidden inside 'their breakfast'.
'eirb' backwards is 'BRIE'.
(Other definitions for brie that I've seen before include "Round soft Fr. cheese" , "Soft and creamy mild white cheese" , "French produce" , "French cream cheese" , "French 8 across" .)