Must shift spot of dirt (4)
I believe the answer is:
smut
'spot of dirt' is the definition.
(smut can mean a piece of dirt)
'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 "Pornography or grime" , "Mildly obscene remarks" , "Soot, dirt, grit" , "Dirt or obscenity" , "blue material" .)