Crossword Genius

Begins with s-sour drink (6,2)

Ross

I believe the answer is:

starts up

'begins' is the definition.
('start up' can be a synonym of 'begin')

's-sour drink' is the wordplay.
'sour' becomes 'tart' ('tart' can be similar in meaning to 'sour').
'drink' becomes 'sup' (I've seen this before).
's'+'tart'+'sup'='STARTS UP'

'with' is the link.

(Another definition for starts up that I've seen is " Gets off the ground".)

I've seen this clue in The Independent.
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