Learner in audience puzzled about classical geometry (9)
I believe the answer is:
euclidean
'about classical geometry' is the definition.
(as in Euclidean geometry)
'learner in audience puzzled' is the wordplay.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'in' indicates putting letters inside.
'puzzled' indicates an anagram.
'audience' anagrammed gives 'eucidean'.
'l' going within 'eucidean' is 'EUCLIDEAN'.
(Other definitions for euclidean that I've seen before include "type of geometry" , "kind of geometry" , "Of an ancient system of geometry" , "of ancient Greek" .)