Crossword Genius

Deaf-ish nans treated, being well looked after (2,4,5)

Ross

I believe the answer is:

ex post facto

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'after' is the definition.
(thesaurus)

'deaf-ish nans treated being well looked' is the wordplay.
I cannot quite understand how this works, but
'being' could be 'ex' (ex is a kind of being) and 'ex' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for ex post facto that I've seen before include "With retrospective action (Latin)" , "retrospectively" .)

I've seen this clue in The Everyman puzzle in The Observer.
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