The most common and most likely answer to this crossword clue is the 4 letter word SHIV
S
H
I
V
More clues leading to the result SHIV
Buddy explains!

The straight part
We know that "weapon" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case weapon = shiv.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 SHIV mean?
SHIV noun- A knife, especially a makeshift one fashioned from something not normally used as a weapon (like a plastic spoon or a toothbrush).
- A particular woody by-product of processing flax or hemp.
- To stab someone with a shiv.
- (by extension) To stab someone with anything not normally used as a stabbing weapon.