Withdraw from a union (6)
I believe the answer is:
secede
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">withdraw from a union</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)