The most common and most likely answer to this crossword clue is the 3 letter word CHI
I've seen this crossword clue in The Times
C
H
I
More clues leading to the result CHI
- Endlessly stylish Greek character
- Cold greeting in foreign letter
- Vital force only somewhat achievable
- X is in when C is out
- Charlie greeting character from abroad
- Greek character taps letters on one
- Greek character's cold greeting
- One character abroad, two would be stylish
- Letter from abroad from Charlie with greeting
- . Computer processor with no power or energy
Buddy explains!

The straight part
We know that "X" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case x = chi.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does CHI mean?
CHI noun- The twenty-second letter of the Classical and Modern Greek alphabets.
- A life force in traditional Chinese philosophy, culture, medicine, etc, related (but not limited) to breath and circulation.
- The Chinese foot, a traditional Chinese unit of length based on the human forearm.
- (Mainland China) The Chinese unit of length standardized in 1984 as 1/3 of a meter.
- (Taiwan) The Taiwanese unit of length standardized as 10/33 of a meter, identical to the Japanese shaku.
- (Hong Kong) The chek or Hong Kong foot, a unit of length standardized as 0.371475 meters.