School too affected by dust (4)
I believe the answer is:
soot
'dust' is the definition.
(thesaurus)
'school too affected by' is the wordplay.
'school' becomes 's'.
'affected' indicates an anagram.
'by' says to put letters next to each other.
'too' anagrammed gives 'oot'.
's'+'oot'='SOOT'
(Other definitions for soot that I've seen before include "Black chimney powder" , "Stuff from chimneys" , "Chimney coating" , "Carbon deposit from burning" , "Carbon black" .)