Only a serf, but he had plenty! (5)
I believe the answer is:
helot
'only a serf but he had plenty' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he had plenty' is the wordplay.
'had plenty' becomes 'lot' (I can't explain this - if you can you should believe this answer much more).
'he'+'lot'='HELOT'
Can you help me to learn more?
(Other definitions for helot that I've seen before include "Greek serf" , "Hotel (anag) - serf" , "He's bound to land" , "Spartan serf" , "Serf of ancient Sparta" .)