A patch liable to emerge from this feature of first floor letters (12,5)

I believe the answer is:

alphabetical order

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'a patch liable to emerge from this feature' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'of first floor letters' is the wordplay.
I cannot really understand how this works, but
'of' could be 'o' and 'o' is found within the answer.
'first' could be 'alpha' and 'alpha' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Guardian.

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