Presumption following country's downfall, naturally (8)
I believe the answer is:
landslip
'downfall naturally' is the definition.
(term for a landslide)
'presumption following country's' is the wordplay.
'presumption' becomes 'lip' (lip can mean presumptuous behaviour).
'following' is a charade indicator (letters next to each other).
'country's' becomes 'land's' ('land' can be a synonym of 'country').
'lip' put after 'lands' is 'LANDSLIP'.
(Other definitions for landslip that I've seen before include "Fall of earth, stones, etc down slope" , "Rockfall" , "Earth movement" , "Avalanche" .)