The most common and most likely answer to this crossword clue is the 7 letter word PISTOLE
I've seen this crossword clue in The Times
P
I
S
T
O
L
E
More clues leading to the result PISTOLE
Buddy explains!

The straight part
We know that "Old coin" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old coin = pistole.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 PISTOLE mean?
PISTOLE noun- A Spanish gold double-escudo coin of the mid-sixteenth century, or any of various gold coins derived from or based on this.