Withdraw officially (6)
I believe the answer is:
secede
'withdraw officially' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for secede that I've seen before include "Formally achieve independence" , "Disaffiliate" , "'Withdraw, break away (6)'" , "Formally withdraw (from federal union)" , "Withdraw (from a federal union or organisation)" .)