Harmful beast one's restrained by common sense (7)
I believe the answer is:
noxious
'harmful' is the definition.
('noxious' is associated in meaning with 'harmful')
'beast one's restrained by common sense' is the wordplay.
'beast' becomes 'ox'.
'one' becomes 'i' (Roman numeral).
'restrained by' means one lot of letters goes inside another.
'common sense' becomes 'nous' (nous is a kind of common sense).
'ox'+'i'='oxi'
'oxi' inserted inside 'nous' is 'NOXIOUS'.
(Other definitions for noxious that I've seen before include "Unwholesome" , "bad for you" , "Harmful, toxic" , "Harmful to health" , "Harmful, poisonous" .)