Close to Mount on United pitch (4)
I believe the answer is:
tone
'pitch' is the definition.
(I know that tone is a type of pitch)
'close to mount on united' is the wordplay.
'close to' says to take the final letters (the 'close' to the word).
'on' means one lot of letters go next to another.
'united' becomes 'one' (as in a phrase like 'we are one').
The last letter of 'mount' is 't'.
't'+'one'='TONE'
(Other definitions for tone that I've seen before include "Theobald Wolf . . . ." , "Quality of a person's voice" , "Bodily firmness" , "A pitch of note" , "Pitch of a voice" .)