Smell is nothing grim (5)
I believe the answer is:
odour
'smell' is the definition.
('odour' can be a synonym of 'smell')
'nothing grim' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'grim' becomes 'dour' ('dour' can be a synonym of 'grim').
'o'+'dour'='ODOUR'
'is' acts as a link.
(Other definitions for odour that I've seen before include "Stink" , "Bouquet" , "Small, aroma" , "Aroma or fragrance" , "A smell, good or bad" .)