Bad mark for a boy getting out of line (5)

I believe the answer is:

stain

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'bad mark' is the definition.
(I've seen this in another clue)

I don't understand the remainder of the clue.

(Other definitions for stain that I've seen before include "Discolouration" , "Blotch on satin" , "Taint" , "Blemish; colour (wood)" , "Black mark for saint" .)

I've seen this clue in the Evening Standard.

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