Crossword Genius

Balances fizzy drink, moving head down (5,2)

Ross

I believe the answer is:

evens up

I'm a little stuck... Click here to teach me more about this clue!

'balances' is the definition.
(evening up is a kind of balancing)

'fizzy drink moving head down' is the wordplay.
I cannot quite see how this works, but
'drink' could be 'sup' (to sup is to drink) and 'sup' is found in the answer.
The remaining letters 'even' 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?

I've seen this clue in The Times.
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