Succeeded in a resolution of ancient case (8)
I believe the answer is:
instance
'case' is the definition.
('instance' can be a synonym of 'case')
'succeeded in a resolution of ancient' is the wordplay.
'succeeded' becomes 's' (genealogical abbreviation).
'in' is an insertion indicator.
'a resolution of' is an anagram indicator (I've seen 'resolution' mean this).
'ancient' is an anagram of 'intance'.
's' put within 'intance' is 'INSTANCE'.
(Other definitions for instance that I've seen before include "An insect (anag.)" , "Example or single occurrence of something" , "Case or example from ancients" , "Exemplify" , "A case or example of something" .)