After instrument goes from cold to hot, it buzzes (6)
I believe the answer is:
hornet
'it buzzes' is the definition.
(hornets make a buzzing sound)
'after instrument goes from cold to hot' is the wordplay.
I cannot really understand how this works, but
'instrument' could be 'horn' (horn is a kind of instrument) and 'horn' is located in the answer.
'from' could be 'e' (I've seen this in other clues) and 'e' is present in the answer.
A single letter 't' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Other definitions for hornet that I've seen before include "Large wasp with vicious sting" , "Troublesome nester" , "Run through" , "A large wasp with serious sting" , "A large wasp with severe sting (6)" .)