Number making heck of a racket in exam (7)
I believe the answer is:
ordinal
'number' is the definition.
(ordinal number is a kind of number)
'heck of a racket in exam' is the wordplay.
'heck of a racket' becomes 'din'.
'in' indicates putting letters inside.
'exam' becomes 'oral' (oral exam is a kind of exam).
'din' inserted into 'oral' is 'ORDINAL'.
'making' acts as a link.
(Other definitions for ordinal that I've seen before include "Numerals such as first and second" , "Relating to position in a sequence" , "Kind of number defining position in series" , "First and second, say" , "Indicating position in sequence - iron lad (anag)" .)