While there are some words that start with "X" in French, they are mostly nouns or verbs:
* Xénon: Xenon (a chemical element)
* Xylophone: Xylophone
* Xérophyte: Xerophyte (a plant adapted to dry environments)
* Xerox: Xerox (a brand of photocopying machines)
* Exécuter: To execute
It's worth noting that some words borrowed from other languages might begin with "X" and function as adjectives, but they are not considered typical French words.