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