Number in school creating a stink (6)
I believe the answer is:
stench
'a stink' is the definition.
('stench' can be a synonym of 'stink')
'number in school' is the wordplay.
'number' becomes 'ten' (ten is a kind of number).
'in' indicates putting letters inside.
'school' becomes 'sch'.
'ten' put within 'sch' is 'STENCH'.
'creating' is the link.
(Other definitions for stench that I've seen before include "Unpleasant odour" , "A really foul smell" , "Terrible smell" , "Strong offensive smell" , "Offensive odour" .)