Part of speech that's hummed out loud (4)
I believe the answer is:
word
'part of speech' is the definition.
(words are parts of what is spoken)
'hummed out loud' is the wordplay.
'hummed' becomes 'whirred'.
'out loud' shows a homophone (sound like).
'whirred' is a homophone of 'WORD'.
'that's' acts as a link.
(Other definitions for word that I've seen before include "Remark; news" , "Puzzle with solutions reading the same down as across" , "See 21" , "Communication; news" , "The Bible is the . . . . of God" .)