Smut in Springfield not unknown? (4)
I believe the answer is:
dust
'smut' is the definition.
(thesaurus)
'springfield not unknown?' is the wordplay.
'springfield' becomes 'dusty' (I've seen this before).
'not' suggests deleting specific letters.
'unknown?' becomes 'y' (algebra).
'dusty' with 'y' taken away is 'DUST'.
'in' acts as a link.
(Other definitions for dust that I've seen before include "Unwanted fine powder" , "Powdered matter" , "Tiny particles of dirt, skin flakes etc" , "Powdery matter" , "Do some housework - we all come to it!" .)