noun
clever person; smart person
Refers to someone who is sharp, sensible, or quick-witted. Often used with a nuance of being shrewd or knowing how to handle situations well.
彼は利口者だから、難しい問題もすぐに解決できる。
He's a clever person, so he can solve difficult problems right away.
あの子は利口者で、一度聞いたことは忘れ
That child is smart and never forgets what they hear once.
利口 is a na-adjective/noun meaning 'clever' or 'sensible', while 利口者 specifically refers to a person who is clever.
Compound of 利口 (rikō, 'clever') and 者 (mono, 'person'). The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning.