Is clientele the correct singular form of the word client?

No, client is a singluar Word the plural of which is clients.

Clientele is a noun, borrowed from the French , meaning the body of people, customers or visitors who frequent a particular venue. The Word is sometimes written in the original form with an accent - clientèle.

Usage: "Although this parlicular client was well-dressed, the shop's clientele generally wore torn jeans".