Crossword Genius

Banknote left in tray emptied (6)

Ross

I believe the answer is:

twenty

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

'banknote' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'left in tray emptied' is the wordplay.
'left' becomes 'went' (I've seen this before).
'in' means one lot of letters goes inside another.
'emptied' suggests removing the centre.
'tray' with its middle removed is 'ty'.
'went' put into 'ty' is 'TWENTY'.

Can you help me to learn more?

(Other definitions for twenty that I've seen before include "Hundredweights to a ton" , "Dozen plus eight" , "Cardinal" , "No longer a teenager" , "One score" .)

I've seen this clue in The Independent and The Times.
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