Cryptic crossword clue
Irregular school lesson in fine class girl missed 9

The most common and most likely answer to this crossword clue is the 9 letter word APERIODIC

I've seen this crossword clue in The Guardian
A
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Buddy explains!

The straight part
We know that "Irregular" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case irregular = aperiodic.
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 APERIODIC mean?

APERIODIC adjective
  1. That does not recur periodically
  2. That does not have a periodic vibration
  3. (stochastic processes, of a state) for which any return to it may occur at irregular times; not periodic.

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