Break away (6)
I believe the answer is:
'break away' is the definition.
(I know that break away can be written as secede)
This is all the clue.
(Other definitions for secede that I've seen before include "Withdraw formally from membership or alliance" , "'Withdraw, break away (6)'" , "Formally withdraw (from federal union)" , "'Withdraw formally, from a political body perhaps (6)'" , "Withdraw from a body, a nation say" .)