Nasty leader deposed, as should be the case (8)
I believe the answer is:
'as should be the case' is the definition.
(as in the rightful heir to the throne)
'nasty leader deposed' is the wordplay.
'nasty' becomes 'frightful' (I've seen this before).
'leader deposed' means to remove the first letter.
'frightful' with its initial letter taken off is 'RIGHTFUL'.
(Other definitions for rightful that I've seen before include "Lawful" , "Legitimate, fitting" , "Having a legally established claim" , "Proper - legitimate" , "Legitimately claimed" .)