The twins' game (4)

I believe the answer is:

snap

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'the twins' game' is the definition.
'snap' can be an answer for 'game' (card game). I am not sure about the 'the twins'' bit.

'twins' game' is the wordplay.
'twins'' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'game' becomes 'nap' (card game).
's'+'nap'='SNAP'

Can you help me to learn more?

(Other definitions for snap that I've seen before include "Photograph - card game" , "type of election" , "Cut" , "Sudden break" , "Speak in a sharp angry tone" .)

I've seen this clue in the Evening Standard.

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