Someone who is employed, or does something, part-time(noun)
A part-time bowler(noun)
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Use "part-timer" in a sentence
"During my first 20 years as a selling writer I was a part-timer."
"Though hired as a part-timer, he went full-time two weeks later, dropping his pursuit of a second degree at Stanford."
"Instead, Ray Wilkins, previously a likable part-timer, has been given free rein to incant his belief in "staying on your feet", while Redknapp continues to fidget and ramble, occasionally sparkling with accidental insight like some tousled piano-playing man‑child genius."