The most common and most likely answer to this crossword clue is the 6 letter word PARANG
I've seen this crossword clue in The Times
P
A
R
A
N
G
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The straight part
We know that "knife" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case knife = parang.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 PARANG mean?
PARANG noun- A short, heavy, straight-edged knife used in Malaysia and Indonesia as a tool and weapon.
- A style of music originating from Trinidad and Tobago, strongly influenced by Venezuelan music.