Mark, showing stress, runs away (5)

I believe the answer is:

stain

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'mark' is the definition.
(I know that mark can be written as stain)

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

(Other definitions for stain that I've seen before include "Black mark for saint" , "Smear" , "Satin (anag.)" , "Discolouration" , "Stigma" .)

I've seen this clue in The Sun.

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