The most common and most likely answer to this crossword clue is the 7 letter word ZODIACS
I've seen this crossword clue in The Guardian
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The straight part
We know that "circular diagrams" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case circular diagrams = zodiacs.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 ZODIACS mean?
ZODIACS noun- The belt-like region of the celestial sphere approximately eight degrees north and south of the ecliptic which include the apparent path of the sun, moon, and visible planets.
- The twelve equal divisions of the zodiacal region into signs or houses, each named for a prominent constellation in the region.
- The ecliptic: the belt-like region of the celestial sphere corresponding to the apparent path of the sun over the course of a year.
- Any of various astrological systems considered similar to the above.
- A circle decorated with the signs of the zodiac.