The most common and most likely answer to this crossword clue is the 6 letter word CREESE
C
R
E
E
S
E
Buddy explains!

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 = creese.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 CREESE mean?
CREESE noun- An Indonesian or Malay dagger with a wavy, or rigid serpentine blade.
- A Moro sword with an asymmetrical blade.
- To stab with a kris.