Don't sit doing nothing! (5,4)

I believe the answer is:

stand idle

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'don't' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'sit doing nothing' is the wordplay.
'sit' becomes 'stand' (I can't justify this - if you can you should believe this answer much more).
'doing nothing' becomes 'idle' (I've seen this in another clue).
'stand'+'idle'='STAND IDLE'

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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