A major player in the field in question: someone who is in the big time(noun)
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Use "big-timer" in a sentence
"There's Grover Norquist, a conservative movement big-timer, joining up with GOProud -- the "national organization of gay conservatives" that served as a CPAC sponsor."
"The first big-timer to step up was Jermaine Dupri, the multiplatinum producer."
"Every big-timer who contributed to this film in some significant way needs to drive out to the desert and hide out for a week or two."