The most common and most likely answer to this crossword clue is the 5 letter word FUSIL
I've seen this crossword clue in The Times
F
U
S
I
L
More clues leading to the result FUSIL
Buddy explains!

The straight part
We know that "Old weapon" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old weapon = fusil.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 FUSIL mean?
FUSIL noun- A bearing of a rhomboidal figure, originally representing a spindle in shape, longer than a heraldic lozenge.
- A light flintlock musket or firelock.
- That can be melted; meltable.
- That has been melted to by heat; liquid, flowing.
- Forged or formed by melting or casting.