The most common and most likely answer to this crossword clue is the 9 letter word BREAKAWAY
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More clues leading to the result BREAKAWAY
Buddy explains!

The straight part
We know that "secession" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case secession = breakaway.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does BREAKAWAY mean?
BREAKAWAY noun- The act of breaking away from something.
- A group of riders which has gone ahead of the peloton.
- A situation in the game where one or more players of a team attack towards the goal of the other team without having any defenders in front of them.
- The act of getting away from one's opponent; the separation of the boxers after a spell of infighting.
- A stampede of animals.
- An animal that breaks away from a herd.
- An eroding steep slope on the edge of a plateau.
- A particular yo-yo trick http//yoyo.wikia.com/wiki/Breakaway.
- A swing dance in which the leader occasionally swings the follower out into an open position.
- Having broken away from a larger unit.
- Capable of breaking off without damaging the larger structure.
- Occurring during or as a result of a breakaway (see Noun)
- (entertainment industry) Enjoying rapid popular success.