The most common and most likely answer to this crossword clue is the 5 letter word SCUDO
S
C
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O
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 = scudo.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 SCUDO mean?
SCUDO noun- A silver coin and unit of currency of various Italian states from the 16th to the 19th centuries.
- A former unit of currency in Malta, now the official currency of the Sovereign Military Order of Malta.
- A unit of currency in 19th-century Bolivia, equal to 16 soles.