Home Counties criminal seen by Cornwall river? That’s not true (9)
I believe the answer is:
falsehood
'that's not true' is the definition.
(a falsehood is false or untrue)
'home counties criminal seen by cornwall river?' is the wordplay.
'home counties' becomes 'SE' (Home Counties in the South East of England).
'criminal' becomes 'hood' (hood is a kind of criminal).
'seen by' means one lot of letters go next to another.
'cornwall river?' becomes 'Fal' (River Fal in Cornwall).
'se'+'hood'='sehood'
'sehood' after 'fal' is 'FALSEHOOD'.
(Other definitions for falsehood that I've seen before include "Pork pie" , "Fictional account" , "It might be hard to swallow" , "Has fooled with lie" , "Untruth" .)