Mark Twain was one creator of characters (10)
I believe the answer is:
typewriter
'creator of characters' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'mark twain was one' is the wordplay.
'mark' becomes 'type' (I've seen this before).
'twain was one' becomes 'writer' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'type'+'writer'='TYPEWRITER'
Can you help me to learn more?
(Other definitions for typewriter that I've seen before include "Machine superseded by the word processor" , "Longest word obtainable using only the top line of a normal keyboard" , "Former secretarial tool" , "creates many characters" , "printing machine" .)