What an adjective modifies (4)
I believe the answer is:
noun
'what an adjective modifies' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for noun that I've seen before include "Substantive word" , "Word used as a name for something or person" , "Part of speech identifying a person, place or thing" , "Word identifying people, things, etc" , "Identifying word (common or proper)" .)