Knock for the mechanic, looking embarrassed (4)

I believe the answer is:

pink

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'knock' is the definition.
(I know that knock can be written as pink)

I can't explain the rest of the clue.

(Other definitions for pink that I've seen before include "Light red - flower" , "Knock (of engine) - garden flower" , "(Shade of) flower" , "Pale red" , "punch" .)

I've seen this clue in the Evening Standard.

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