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