Quietly replacing youth's mat showing dirtiness (8)
I believe the answer is:
impurity
'dirtiness' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for impurity that I've seen before include "Unclean constituent" , "Contamination" , "Contaminant" , "pollution" .)