Mark and I interrupt Stanley (5)
I believe the answer is:
stain
'mark' is the definition.
(synonyms)
I cannot 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 "Blot" , "Mark with colour, dirt" , "Unsightly mark" , "Blemish; colour (wood)" , "Satin (anag.)" .)