Withdraw formally (6)
I believe the answer is:
secede
'withdraw formally' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for secede that I've seen before include "Formally withdraw (from federal union)" , "Withdraw (politically)" , "'Withdraw formally, from a political body perhaps (6)'" , "Formally achieve independence" , "Withdraw from a body, a nation say" .)