A number making a noise in an examination (7)
I believe the answer is:
ordinal
'a number' is the definition.
(I know that ordinal is a type of number)
'a noise in an examination' is the wordplay.
'a noise' becomes 'din' (din can mean a loud noise or disturbance).
'in' indicates putting letters inside.
'an examination' becomes 'oral' (oral examination is a kind of examination).
'din' inserted within 'oral' is 'ORDINAL'.
'making' acts as a link.
(Other definitions for ordinal that I've seen before include "Roman Catholic service book" , "Number in a sequence" , "Number such as first or second" , "Possibly first or second" , "Of position in series; service book (hist.)" .)