George Eliot was one to write, terribly mean (3,4)
I believe the answer is:
pen name
'george eliot' is the definition.
Both the definition and answer are singular nouns.
Maybe there's an association between them I don't understand?
'one to write terribly mean' is the wordplay.
'one to write' becomes 'pen' (I've seen this in another clue).
'terribly' is an anagram indicator.
'mean' anagrammed gives 'name'.
'pen'+'name'='PEN-NAME'
'was' acts as a link.
Can you help me to learn more?
(Other definitions for pen name that I've seen before include "such as Mark Twain" , "Author's pseudonym" , "Nom-de-plume" , "Writer's pseudonym" , "Nom de plume" .)