Some not present presumably? That's somewhat stupid (3,3,5)

I believe the answer is:

not all there

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

'some not present presumably?' is the wordplay.
'some not' becomes 'not'.
'present presumably?' becomes 'allthere' (I can't justify this - if you can you should give a lot more credence to this answer).
'not'+'allthere'='NOT ALL THERE'

'that's' is the link.

Can you help me to learn more?

I've seen this clue in The Independent.

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