Smell that's nothing serious (5)
I believe the answer is:
odour
'smell' 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'
'that's' acts as a link.
(Other definitions for odour that I've seen before include "Small, aroma" , "pervasive quality" , "Whiff" , "Regard" , "Smell, redolence" .)