A formal written statement of relinquishment (6)
I believe the answer is:
waiver
'a formal written statement of relinquishment' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for waiver that I've seen before include "Statement giving up a claim" , "Document surrendering rights" , "Disclaimer of a legal right" , "Letting go right" , "Formal for going of legal right or claim" .)