Reeve's role, originally, over staff (8)
I believe the answer is:
superman
'reeve's' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'role originally over staff' is the wordplay.
'role originally' becomes 'super' (I can't explain this - if you can you should believe this answer much more).
'over' says to put letters next to each other.
'staff' becomes 'man' (as in staffing or manning a particular place).
'super'+'man'='SUPERMAN'
Can you help me to learn more?
(Other definitions for superman that I've seen before include "Nietzsche hero; comic-strip character" , "Nietzche's (or fictional) hero" , "Nietzschean or cartoon hero" , "one with great abilities" , "Clark Kent" .)