Note changed in pitch (4)
I believe the answer is:
tone
'pitch' is the definition.
(I know that tone is a type of pitch)
'note changed' is the wordplay.
'changed' indicates an anagram.
'note' with letters rearranged gives 'TONE'.
'in' acts as a link.
(Other definitions for tone that I've seen before include "accent" , "Quality or character of voice" , "Pitch (sound)" , "Sound signal on a telephone" , "Musical sound of note" .)