The most common and most likely answer to this crossword clue is the 7 letter word ROUNDEL
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The straight part
We know that "Circle" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case circle = roundel.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 ROUNDEL mean?
ROUNDEL noun- Anything having a round form; a round figure; a circle.
- A roundelay or rondelay.
- A small circular shield, sometimes not more than a foot in diameter, used by soldiers in the fourteenth and fifteenth centuries.
- A circular spot; a charge in the form of a small coloured circle.
- A circular insignia painted on an aircraft to identify its nationality or service.
- A bastion of a circular form.