A mark that must be shifted (4)
I believe the answer is:
smut
'a mark' is the definition.
(thesaurus)
'must be shifted' is the wordplay.
'be shifted' is an anagram indicator.
'must' with letters rearranged gives 'SMUT'.
'that' is the link.
(Other definitions for smut that I've seen before include "Mildly obscene remarks" , "Soot flake; cereal disease" , "Speck of soot" , "Small soot flake or indecent talk" , "Indecent talk - black mark" .)