Offensive at first, grim smell (5)
I believe the answer is:
odour
'smell' is the definition.
(I know that smell can be written as odour)
'offensive at first grim' is the wordplay.
'at first' suggests taking the first letters.
'grim' becomes 'dour' (synonyms).
The initial letter of 'offensive' is 'o'.
'o'+'dour'='ODOUR'
(Other definitions for odour that I've seen before include "reputation" , "Smell; repute" , "for the nose to detect" , "pervasive quality" , "Stink" .)