One very good at reforming the bad? (3)

I believe the answer is:

dab

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'one very good at' is the definition.
The definition and answer can be both quantities as well as being singular nouns.
Maybe there's an association between them I don't understand?

'reforming the bad?' is the wordplay.
'reforming' indicates an anagram.
'bad' with letters rearranged gives 'DAB'.

Can you help me to learn more?

(Other definitions for dab that I've seen before include "Bad kind of fish" , "A small fish - bad one" , "[SOMETHING FISHY]" , "Expert; light touch" , "Bad sort of small fish" .)

I've seen this clue in the Evening Standard.

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