Part of speech some deliver brilliantly (4)
I believe the answer is:
verb
'part of speech' is the definition.
(verb is a kind of part of speech)
'some deliver brilliantly' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'VERB' is hidden in the letters of 'deliver brilliantly'.
(Other definitions for verb that I've seen before include "Word such as run, make, or do" , "Word that denotes an action or a state" , "A word that conveys action" , "love, for example" , "Predicate of a sentance (4)" .)