Forbidding smell, mostly (4)
I believe the answer is:
dour
'forbidding' is the definition.
(I know that forbidding can be written as dour)
'smell mostly' is the wordplay.
'smell' becomes 'odour' (synonyms).
'mostly' means to remove the first letter (1).
'odour' with its first letter taken away is 'DOUR'.
(Other definitions for dour that I've seen before include "Of appearance, harsh and forbidding" , "Gloomy or sullen" , "Obstinate - grim" , "Morose, sour" , "Gloomy-looking, stern" .)