NONE!
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.
The most common word of that form is "relax". Others are "radix" and "redux".
Calyx, codex and cylix are 5 letter words. They begin with c and end with x.
Twix
There is no synonym for awesome beginning with the letter x.
x-ray
What is a word that starts with X meaning nice or playful
thorax
Flax.
cortex
Quincunx is a word. It ends with X.