Irregular school lesson in fine class girl missed (9)
I believe the answer is:
aperiodic
'irregular' is the definition.
(not occurring regularly or periodically)
'school lesson in fine class girl missed' is the wordplay.
I cannot really see how this works, but
'lesson' could be 'period' and 'period' is located in the answer.
'fine' could be 'ai' (I've seen this in other clues) and 'ai' is present in the remaining letters.
A single letter 'c' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for aperiodic that I've seen before include "at irregular intervals" , "without regular metre?" .)