How are you drinking flat soda? (5-2)

I believe the answer is:

seven-up

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'how are you drinking flat soda?' is the definition.
I can't tell whether this definition defines the answer.

'drinking flat soda?' is the wordplay.
'drinking' indicates putting letters inside (inserted letters are consumed or drunk).
'flat' becomes 'even' (level).
'soda?' becomes 'sup' (I am not sure about this - if you are sure you should believe this answer much more).
'even' inserted inside 'sup' is 'SEVEN-UP'.

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