The most common and most likely answer to this crossword clue is the 6 letter word OBELUS
I've seen this crossword clue in The Times
O
B
E
L
U
S
More clues leading to the result OBELUS
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The straight part
We know that "dagger" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dagger = obelus.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 OBELUS mean?
OBELUS noun- A symbol resembling a horizontal line (–), sometimes together with one or two dots (for example, ⨪ or ÷), which was used in ancient manuscripts and texts to mark a word or passage as doubtful or spurious, or redundant; an obelisk.
- A dagger symbol (†), which is used in printed matter as a reference mark to refer the reader to a footnote, marginal note, etc.; beside a person's name to indicate that the person is deceased; or beside a date to indicate that it is a person's death date; an obelisk.