Part of speech never bettered, but only part (4)
I believe the answer is:
verb
'part of speech' is the definition.
(verb is a kind of part of speech in grammar)
'never bettered but only part' is the wordplay.
'but only part' says the answer is hidden in the clue.
'VERB' is hidden within 'never bettered'.
(Other definitions for verb that I've seen before include "love, for example" , "Word such as run, make, or do" , "Word that denotes an action or a state" , "Part of speech" , "The predicate of a sentence" .)