Mark and Stan getting out of line (5)
I believe the answer is:
stain
'mark' is the definition.
(I know that mark can be written as stain)
'stan getting out of line' is the wordplay.
'getting' means one lot of letters goes inside another.
'out of line' becomes 'i' (I can't justify this - if you can you should believe this answer much more).
'stan' enclosing 'i' is 'STAIN'.
'and' acts as a link.
Can you help me to learn more?
(Other definitions for stain that I've seen before include "Discolouration" , "Penetrative dye; mark" , "Black mark for saint" , "Blemish; colour (wood)" , "Unsightly mark" .)