Cereal in a double-length, renovated barn (3-4)

I believe the answer is:

all-bran

I'm a little stuck... Click here to teach me more about this clue!

'cereal' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'a double-length renovated barn' is the wordplay.
'double' indicates the doubling of some letters.
'length' becomes 'l' (abbreviation in geometry).
'renovated' is an anagram indicator.
'l' duplicated is 'll'.
'barn' is an anagram of 'bran'.
'a'+'ll'+'bran'='ALL-BRAN'

'in' is the link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.