Unchanging in pitch (8)
I believe the answer is:
monotone
'unchanging in pitch' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for monotone that I've seen before include "Flat and unchanging in pitch" , "Dull delivery" , "Utterance at a single unvaried level" , "Sound of unchanging pitch" , "Sound of unvarying pitch" .)