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