More stupefied son with more speed (7)

I believe the answer is:

spacier

'more stupefied' is the definition.
(spacey can mean unable to think clearly)

'son with more speed' is the wordplay.
'son' becomes 's'.
'with more speed' becomes 'pacier'.
's'+'pacier'='SPACIER'

I've seen this clue in The Telegraph.

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