The most common and most likely answer to this crossword clue is the 10 letter word HALF-CROWN
I've seen this crossword clue in The Guardian
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More clues leading to the result HALF-CROWN
Buddy explains!
The straight part
We know that "old piece of silver" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old piece of silver = half-crown.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.
Crossword Answer definitions
HALF-CROWN noun- A pre-decimal coin used in Britain and Ireland, equivalent to 30 pence or two shillings and six pence