Part of office really shows breakfast food (6)
I believe the answer is:
cereal
'breakfast food' is the definition.
(cereal is a kind of breakfast food)
'part of office really shows' is the wordplay.
'part of' indicates a hidden word.
'CEREAL' is hidden in the letters of 'office really shows'.
(Other definitions for cereal that I've seen before include "Rye, for example" , "output from harvest?" , "Breakfast food made from grain" , "Breakfast dish" , "Something for brekkie" .)