A part of speech (6)
I believe the answer is:
adverb
'a part of speech' is the definition.
(adverb is a kind of part of speech)
This is the entire clue.
(Other definitions for adverb that I've seen before include "for example, determinedly" , "Verb qualifier" , "Modifying word (gram.)" , "Word modifying another" , "Word that qualifies an action" .)