Cereal for everyone kept in barrel (5)
I believe the answer is:
durum
'cereal' is the definition.
(I know that durum is a type of wheat)
'for everyone kept in barrel' is the wordplay.
'for everyone' becomes 'U' (Universal film certificate).
'kept in' means one lot of letters goes inside another.
'barrel' becomes 'drum' (synonyms).
'u' put inside 'drum' is 'DURUM'.
(Other definitions for durum that I've seen before include "Kind of wheat used to make pasta" , "Wheat variety" , "Hard type of wheat" , "Kind of hard wheat" , "Type of flour used for pasta" .)