We don't have synonyms for seceding yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are seeming, seedling and preceding.
(v) :
(intransitive) To split from or to withdraw from membership of a political union, an alliance or an organisation.
(v) :
(transitive, uncommon) To split or to withdraw one or more constituent entities from membership of a political union, an alliance or an organisation.
(v) :
(intransitive) To split from or to withdraw from membership of a political union, an alliance or an organisation.
(v) :
(transitive, uncommon) To split or to withdraw one or more constituent entities from membership of a political union, an alliance or an organisation.