In breaking a rule, a doctor will not move fast (6)

I believe the answer is:

lumber

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'will not move fast' is the definition.
I can't tell whether this definition defines the answer.

'in breaking a rule a doctor' is the wordplay.
'in' is an insertion indicator.
'breaking' indicates anagramming the letters.
'a doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'rule' anagrammed gives 'luer'.
'luer' enclosing 'mb' is 'LUMBER'.

Can you help me to learn more?

(Other definitions for lumber that I've seen before include "With difficulty move" , "Move awkwardly; useless things" , "Wood for building - saddle" , "articles in storage" , "Burden with an unwanted task" .)

I've seen this clue in the Evening Standard.

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