Crossword Genius

Quickly prepare large drink on time (4,2)

Ross

I believe the answer is:

toss up

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'quickly prepare' is the definition.
I can't judge whether this definition defines the answer.

'large drink on time' is the wordplay.
'large' becomes 'os' (abbreviation for oversize).
'drink' becomes 'sup' (to sup is to drink).
'on' says to put letters next to each other.
'time' becomes 't' (abbreviation).
'os'+'sup'='ossup'
'ossup' put after 't' is 'TOSS-UP'.

Can you help me to learn more?

(Other definitions for toss up that I've seen before include "Spin a coin - prepare food quickly" , "unpredictable result" , "It's an even chance" , "It's as likely as not" , "close thing" .)

I've seen this clue in The Guardian.
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