One Gentleman of Verona, please, out of costume (9)
I believe the answer is:
ignoramus
'one gentleman' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for ignoramus that I've seen before include "Stupid person" , "One lacking knowledge" , "Dunce" , "A know-nothing" , "At the present time" .)