The most common and most likely answer to this crossword clue is the 8 letter word OSTRACON
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The straight part
We know that "voting tablet" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case voting tablet = ostracon.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 OSTRACON mean?
OSTRACON noun- A piece of pottery or stone, usually broken off from a vase or other earthenware vessel, especially one used to cast a vote during the Ancient Greek process of ostracism.