Must deal with dirt (4)
I believe the answer is:
smut
'dirt' is the definition.
(I've seen this before)
'must deal with' is the wordplay.
'deal with' indicates anagramming the letters (deal with can mean to organise or sort out).
'must' anagrammed gives 'SMUT'.
(Other definitions for smut that I've seen before include "Obscene word(s)" , "Small bit of soot" , "Sooty matter" , "Dirt or pornography" , "Indecent stuff" .)