A number making a noise during examination (7)
I believe the answer is:
ordinal
'a number' is the definition.
(ordinal is a kind of number)
'a noise during examination' is the wordplay.
'a noise' becomes 'din' (din is a kind of noise).
'during' indicates putting letters inside.
'examination' becomes 'oral' (oral is a kind of examination).
'din' placed into 'oral' is 'ORDINAL'.
'making' is the link.
(Other definitions for ordinal that I've seen before include "Roman Catholic service book" , "First and second, say" , "Maybe first" , "Indicating position in sequence - iron lad (anag)" , "E.g. first" .)