The most common and most likely answer to this crossword clue is the 3 letter word ECU
E
C
U
Buddy explains!
The straight part
We know that "coin of old" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case coin of old = ecu.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 ECU mean?
ECU noun- The European Currency Unit (symbol ₠), a currency used in the European Community before the euro.