The most common and most likely answer to this crossword clue is the 6 letter word MUSKET
I've seen this crossword clue in Financial Times
M
U
S
K
E
T
More clues leading to the result MUSKET
- King, Eastern, accommodated in requirement for outdated gun
- Fording Welsh river, came across an old firearm
- Need to conceal knife outside as weapon
- Old banger, American truck finally impounded by police
- Old firearm London police found round river
- Weapon made by space technology promoter and alien
- . Sound of Arabian city's old weapon
- Old weapon’s missing part kept in condition
- Satisfied about King supporting America’s weapon
Buddy explains!

The straight part
We know that "an old firearm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an old firearm = musket.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 MUSKET mean?
MUSKET noun- A kind of firearm formerly carried by the infantry of an army, originally fired by means of a match, or matchlock, for which several mechanical appliances (including the flintlock, and finally the percussion lock) were successively substituted; ultimately superseded by the rifle.
- A male Eurasian sparrowhawk.