Dirty article sat on by relation (7)
I believe the answer is:
unclean
'dirty' is the definition.
('unclean' can be a synonym of 'dirty')
'article sat on by relation' is the wordplay.
'article' becomes 'an' (indefinite article).
'sat on by' says to put letters next to each other.
'relation' becomes 'uncle' (uncle is a kind of relation**).
'an' after 'uncle' is 'UNCLEAN'.
(Other definitions for unclean that I've seen before include "Dirty, needs a wash" , "(Ritually) tainted" , "taboo" , "Defiled" , "Dirty, ritually impure" .)