Utter chaos at the end on mountain (5)
I believe the answer is:
speak
'utter' is the definition.
(I know that utter can be written as speak)
'chaos at the end on mountain' is the wordplay.
'at the end' suggests the final letters.
'on' means one lot of letters go next to another.
'mountain' becomes 'peak' (the peak of a mountain).
The last letter of 'chaos' is 's'.
's'+'peak'='SPEAK'
(Other definitions for speak that I've seen before include "Utter words" , "Say something" , "Orate" , "give a lecture" , "Converse" .)