Least clever son most humble (7)

I believe the answer is:

slowest

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'least clever' is the definition.
I can't tell whether this defines the answer.

'son most humble' is the wordplay.
'son' becomes 's'.
'most humble' becomes 'lowest' ('low' can be similar in meaning to 'humble').
's'+'lowest'='SLOWEST'

Can you help me to learn more?

(Other definitions for slowest that I've seen before include "Last in race" , "Most stupid" , "Least fast" .)

I've seen this clue in The Telegraph.

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