None left - that's OK (8)

I believe the answer is:

allright

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'none left' is the definition.
(I've seen this before)

'that's ok' is the wordplay.
'that' becomes 'all' (I can't justify this - if you can you should believe this answer much more).
'ok' becomes 'right' (I've seen this before).
'all'+'right'='ALLRIGHT'

Can you help me to learn more?

(Other definitions for allright that I've seen before include "Very well" , "None left" .)

I've seen this clue in The Irish News.

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