Numbers racket discovered in spoken exams (8)
I believe the answer is:
ordinals
'numbers' is the definition.
(ordinal is a kind of number)
'racket discovered in spoken exams' is the wordplay.
'racket' becomes 'din' (both can mean a loud noise or disturbance).
'discovered in' indicates putting letters inside.
'spoken exams' becomes 'orals' (I've seen this in another clue).
'din' placed into 'orals' is 'ORDINALS'.
(Other definitions for ordinals that I've seen before include "Numbers" , "Figures indicating position" .)