In short, he certainly wasn't well up in Roman history (4)
I believe the answer is:
pleb
'in short he certainly wasn't well up in roman history' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is all the clue.
Can you help me to learn more?
(Other definitions for pleb that I've seen before include "Term used for a member of the lower classes" , "Person of vulgar tastes (colloq.)" , "Uncultivated type" , "Commoner" , "Boor" .)