Smacker that may leave mouth discoloured (4)
I believe the answer is:
quid
'smacker that may leave mouth discoloured' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
This is all the clue.
Can you help me to learn more?
(Other definitions for quid that I've seen before include "Bit of greens" , "- - pro quo" , "One pound sterling" , "Pound - lump of chewing tobacco" , "Pound sterling (slang)" .)