One proverbially cool copper leaves behind a burden (6)
I believe the answer is:
cumber
'a burden' is the definition.
(thesaurus)
'one proverbially cool copper leaves behind' is the wordplay.
I cannot quite see how this works, but
'copper' could be 'cu' (chemical symbol) and 'cu' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for cumber that I've seen before include "Hinder" , "Impede" , "Get in the way of" .)