Disown note with negative tone lacking weight (6)
I believe the answer is:
renege
'disown' is the definition.
(thesaurus)
'note with negative tone lacking weight' is the wordplay.
I cannot quite see how this works, but
'note' could be 're' (as in do-re-mi) and 're' is found within the answer.
'negative' could be 'neg' (short for photographic negative) and 'neg' is located in the answer.
'tone' could be 'e' and 'e' is found in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for renege that I've seen before include "In card games, revoke" , "Fail to follow suit or fulfil a promise" , "Go back (on promise etc)" , "Fail to carry out a commitment" , "South American cowboy" .)