Some abstemious luminaries in Number Ten? Hardly! (4)

I believe the answer is:

slum

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'number ten? hardly' is the definition.
I can't tell whether this defines the answer.

'some abstemious luminaries in' is the wordplay.
'in' says the answer is hidden in the clue.
'SLUM' is hidden within 'some abstemious luminaries'.

Can you help me to learn more?

(Other definitions for slum that I've seen before include "Over-crowded run-down city area" , "Squalid neighbourhood" , "Area of bad housing" , "Run-down housing area" , "Ghetto" .)

I've seen this clue in The Independent.

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