It may be common or proper (4)
I believe the answer is:
noun
'it may be common or proper' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for noun that I've seen before include "Category of word" , "Part of speech identifying a person, place or thing" , "Naming word (grammar)" , "Content word for person, place, thing etc" , "Identifying word (common or proper)" .)