The most common and most likely answer to this crossword clue is the 4 letter word TEIN
T
E
I
N
Buddy explains!

The straight part
We know that "Currency piece" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case currency piece = tein.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 TEIN mean?
TEIN noun- A currency unit of Uzbekistan, one hundredth of a som.
- A former currency unit of Kazakhstan, one hundredth of a tenge.