Must shift stain (4)
I believe the answer is:
smut
'stain' is the definition.
(I know that smut is a more specific form of the action stain)
'must shift' is the wordplay.
'shift' indicates anagramming the letters.
'must' with letters rearranged gives 'SMUT'.
(Other definitions for smut that I've seen before include "Soot flake or sexual innuendo" , "Indecent items or talk" , "Dirty jokes - black mark" , "(Speck) of dirt" , "Small soot flake or indecent talk" .)