One's drunk at a formal occasion, so totally sozzled having quaffed last of tequila (5,5)

I believe the answer is:

loyal toast

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'formal occasion so totally sozzled having quaffed last of tequila' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'one's drunk at a' is the wordplay.
I cannot quite understand how this works, but
'one's' could be 'as' (a thing is one thing) and 'as' is located in the answer.
'a' is within the answer.
This explanation may well be incorrect...

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I've seen this clue in The Guardian.

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