Stink is offensive at first, and grim (5)
I believe the answer is:
odour
'stink' is the definition.
(I've seen this before)
'offensive at first and grim' is the wordplay.
'at first' indicates taking the first letters.
'and' says to put letters next to each other.
'grim' becomes 'dour' ('dour' can be a synonym of 'grim').
The initial letter of 'offensive' is 'o'.
'o'+'dour'='ODOUR'
'is' is the link.
(Other definitions for odour that I've seen before include "Regard" , "Olfactory sensation" , "reputation" , "for the nose to detect" , "Bouquet" .)