cortex
STIX !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Could be a four letter word that starts with s and ends with x
A four letter word beginning with C and ending with X is crux.
Five letter words starting with C and ending with X are calyx and codex.
phoenix?
Pheonix
The word faux has a silent x, pronounced [foh].
travaux
x-ray
Exit, X-ray
x-ray
Try flux or flax; latter is healthier.