Moment before new seed splits (7)
I believe the answer is:
Here is my best explanation:
'splits' is the definition.
(seceding is a kind of splitting)
'moment before new seed' is the subsidiary indication.
'moment' becomes 'sec'.
'before' says to put letters next to each other.
'new' indicates an anagram.
'seed' is an anagram of 'edes'.
(Other definitions for secedes that I've seen before include "Withdraws from an organization" and "Breaks away".)