What a muffler will do? (6)

I believe the answer is:

stifle

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'do?' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for stifle that I've seen before include "Muffle, suppress" , "Gag" , "Have difficulty breathing" , "Curb" , "Smother - 6 down (anag)" .)

I've seen this clue in the Evening Standard.

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