(a)independent, main(of a clause) capable of standing syntactically alone as a complete sentence“the main (or independent) clause in a complex sentence has at least a subject and a verb”
(s)chief, main, primary, principal, mastermost important element“the chief aim of living”, “the main doors were of solid glass”, “the principal rivers of America”, “the principal example”, “policemen were primary targets”, “the master bedroom”, “a master switch”