After shortening, deliver best part of speech (4)
I believe the answer is:
verb
'part of speech' is the definition.
(verb is a kind of part of speech in grammar)
'after shortening deliver best' is the wordplay.
'after shortening' says the answer is hidden in the clue (words in the clue must be shortened down).
'VERB' is hidden in the letters of 'deliver best'.
(Other definitions for verb that I've seen before include "The predicate of a sentence" , "Type of word indicating action" , "Word such as run, make, or do" , "Old-fashioned art, for example" , "A word that conveys action" .)