Disgusting substance found on a Welshman’s overalls (9)
I believe the answer is:
dungarees
'overalls' is the definition.
(I have seen 'Durable overalls ' mean 'dungarees' so perhaps 'overalls' could also mean 'dungarees')
'disgusting substance found on a welshman's' is the wordplay.
'disgusting substance' becomes 'dung' (excrement).
'found on' means one lot of letters go next to another.
'welshman' becomes 'rees' (common Welsh surname).
'dung'+'a'+'rees'='DUNGAREES'
(Other definitions for dungarees that I've seen before include "Overall" , "Durable overalls" , "Worker's trousers with a bib" , "Hard-wearing work trousers with a bib" , "Overalls of cotton fabric" .)