‘100 + 1 + 100 + 0? Not unknown' - Roman senator (6)
I believe the answer is:
cicero
'roman senator' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'100 1 100 0? not unknown' is the wordplay.
'c00 c c00' becomes 'cic' (I am not sure about this - if you are sure you should believe this answer much more).
'0?' becomes 'zero' ('zero' can be a synonym of '0').
'not' is a deletion indicator.
'unknown' becomes 'z' (algebra).
'zero' with 'z' taken away is 'ero'.
'cic'+'ero'='CICERO'
Can you help me to learn more?
(Other definitions for cicero that I've seen before include "Roman orator, author" , "Roman orator; type size above pica" , "Philosopher" , "Roman lawyer" , "wartime spy" .)