Instrument's initial temperature increases which is welcome (5)
I believe the answer is:
hello
'is welcome' is the definition.
(both are greetings)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for hello that I've seen before include "Greeting - celebrity magazine" , "A salutation" , "I say" , "Surprised cry" , "Welcome" .)