In Rome, old-timers were common (5)
I believe the answer is:
plebs
'in rome old-timers' is the definition.
The definition and answer can be both people as well as being plural nouns.
Perhaps there's an association between them I don't understand?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for plebs that I've seen before include "Common people" , "The masses!" , "Lower classes (derog.)" .)