Crossword Genius

After son moves back, fizzy drink settles (5,2)

Ross

I believe the answer is:

evens up

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

'after son moves' is the definition.
The definition and answer can be both to do with motion as well as being verbs in their -s form.
Perhaps you can see a link between them that I don't see?

'back fizzy drink settles' is the wordplay.
I cannot really understand 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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