Something chewed, filled with brilliant yellow sauce (7)
I believe the answer is:
custard
'yellow sauce' is the definition.
(I've seen this before)
'something chewed filled with brilliant' is the wordplay.
'something chewed' becomes 'cud' (partly digested food that is rechewed).
'filled with' means one lot of letters goes inside another.
'brilliant' becomes 'star' (star can mean brilliant eg star performer).
'cud' placed around 'star' is 'CUSTARD'.
(Other definitions for custard that I've seen before include "Sweet sauce made with milk and eggs" , "Type of apple" , "Yellow mixture" , "Dessert sauce" , "-- Pie; -- apple" .)