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 wordplay.
'moment' becomes 'sec'.
'before' means one lot of letters go next to another.
'new' indicates anagramming the letters.
'seed' is an anagram of 'edes'.
(Other definitions for secedes that I've seen before include "Withdraws from an organization", "Breaks away" and "makes a withdrawal".)