Mark, showing stress, runs away (5)
I believe the answer is:
stain
'mark' is the definition.
(I know that mark can be written as stain)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for stain that I've seen before include "Black mark for saint" , "Smear" , "Satin (anag.)" , "Discolouration" , "Stigma" .)