Smell's nothing serious (5)
I believe the answer is:
odour
'smell's' is the definition.
('odour' can be a synonym of 'smell')
'nothing serious' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'serious' becomes 'dour' (I've seen this in other clues).
'o'+'dour'='ODOUR'
(Other definitions for odour that I've seen before include "for the nose to detect" , "Distinctive smell" , "Smell; repute" , "Perfume" , "Stink" .)