Mark and I interrupt Stan (5)
I believe the answer is:
stain
'mark' is the definition.
(synonyms)
'i interrupt stan' is the wordplay.
'interrupt' indicates putting letters inside.
'i' inserted inside 'stan' is 'STAIN'.
'and' acts as a link.
(Other definitions for stain that I've seen before include "Spot - shame" , "Satin (anag.)" , "Smear" , "Discoloration" , "Black mark for saint" .)