Smell? Nothing serious (5)
I believe the answer is:
odour
'smell?' is the definition.
(I know that smell can be written as odour)
'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 "Bouquet" , "reputation" , "Aroma, smell" , "A smell, good or bad" , "Perfume" .)