Popular breakfast time serial, say (6)

I believe the answer is:

cereal

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'popular breakfast time' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'serial say' is the wordplay.
'say' indicates a 'sounds like' (homophone) clue.
'serial' is a homophone of 'CEREAL'.

Can you help me to learn more?

(Other definitions for cereal that I've seen before include "Eg, wheat, rice" , "Contents of breakfast bowl" , "11 food, maybe" , "Common part of breakfast" , "Grass producing an edible grain" .)

I've seen this clue in the Evening Standard.

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