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 judge whether this definition defines the answer.

'drinking flat soda?' is the wordplay.
'drinking' is an insertion indicator (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 give a lot more credence to this answer).
'even' going into 'sup' is 'SEVEN-UP'.

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