Old-timer developing aneurism (8)
I believe the answer is:
Here is my best explanation:
'old-timer' is the definition.
Both the definition and answer are singular nouns.
Maybe they are linked in a way I don't understand?
'developing aneurism' is the wordplay.
'developing' is an anagram indicator.
'aneurism' anagrammed gives 'SUMERIAN'.
(Other definitions for sumerian that I've seen before include "One who lived long ago", "one of old people (like 24A)" and "Language".)