The most common and most likely answer to this crossword clue is the 5 letter word CARET
I've seen this crossword clue in The Times
C
A
R
E
T
More clues leading to the result CARET
- Bordeaux leaking litres, a sign something's missing
- Sign of something missing? React violently
- The mark of an editor, leader in Courier given award once
- Carpet Power’s removed mark
- Mark on script what could be picked up as incentive
- No room for litres of red wine? Mark suggests something could be squeezed in
- Ahead of time, be concerned it shows where text is lacking
- Mark for insertion in Building Act to cover engineers
- A sign of something missing? Trouble with that hat going astray
- React poorly to keyboard symbol
Buddy explains!

The straight part
We know that "a sign something's missing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a sign something's missing = caret.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 CARET mean?
CARET noun- A mark ⟨ ‸ ⟩ used by writers and proofreaders to indicate that something is to be inserted at that point.
- An indicator, often a blinking line or bar, indicating where the next insertion or other edit will take place. Also called a cursor.
- A circumflex, ⟨ ^ ⟩.
- A háček, ⟨ ˇ ⟩.
- A kind of turtle, the hawksbill (Eretmochelys imbricata).