Old grim smell (5)
I believe the answer is:
odour
'smell' is the definition.
(I know that smell can be written as odour)
'old grim' is the wordplay.
'old' becomes 'o' (abbreviation).
'grim' becomes 'dour' (synonyms).
'o'+'dour'='ODOUR'
(Other definitions for odour that I've seen before include "Smell, scent" , "Aroma, smell" , "Regard" , "Stench" , "Pong" .)