Stupid bears smell beastly (8)
I believe the answer is:
inhumane
'beastly' is the definition.
('inhumane' can be similar in meaning to 'beastly')
'stupid bears smell' is the wordplay.
'stupid' becomes 'inane' (I've seen this before**).
'bears' means one lot of letters goes inside another.
'smell' becomes 'hum' (I have seen 'Unpleasant smell ' mean 'hum' so perhaps 'smell' could also mean 'hum').
'inane' enclosing 'hum' is 'INHUMANE'.
(Other definitions for inhumane that I've seen before include "Heartless" , "Not compassionate" , "Callous" , "Cruel" .)