The most common and most likely answer to this crossword clue is the 3 letter word TEN
I've seen this crossword clue in The Daily Telegraph
T
E
N
More clues leading to the result TEN
- How many Christie started with, then first of house-guests is lost
- Headgear which is worn by everyone in English cricket side
- Number in catch returned
- Number of lions eaten by 19, contrarily
- Figure after tax returns
- X may be the one regularly going missing
- Somewhat incompetent, all fingers and thumbs?
- Film I love in extended version
- Catch over sixteen ounces is worth half a score
- Protections for Western head that is retaining engineer — quite practical
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 = ten.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 TEN mean?
TEN noun- A set or group with ten elements.
- A card in a given suit with a value of ten.
- A denomination of currency, such as a banknote, with a value of ten units. See also tenner.
- A perfect specimen, a physically attractive person.
- A high level of intensity.
- The act of rowing ten strokes flat out.
- The number occurring after nine and before eleven, represented in Arabic numerals (base ten) as 10 and in Roman numerals as X.