Crossword Genius

Wine's two appealing qualities when knocked back (4)

Ross

I believe the answer is:

asti

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'wine's' is the definition.
(Italian wine)

'two appealing qualities when knocked back' is the wordplay.
I cannot quite understand how this works, but
'when' could be 'as' (eg 'when I walked' means 'as I walked') and 'as' is found in the answer.
The remaining letters 'ti' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for asti that I've seen before include "Sparkling wine named after an Italian Piedmont town" , "18" , "Alcohol" , "Bubbly wine" , "Fizzy wine" .)

I've seen this clue in The Times.
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