Embellished walls on upper flat (3-1)

I believe the answer is:

one-d

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'flat' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'embellished walls on upper' is the wordplay.
'walls' suggests removing the centre (the walls are the letters on each edge).
'upper' indicates an anagram (I've seen 'up' mean this (letters get chucked up)).
'embellished' with its middle taken out is 'ed'.
'ed'+'on'='edon'
'edon' is an anagram of 'ONE-D'.

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

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