A bad smell, Doctor! A frightening thing (1-4)
I believe the answer is:
a-bomb
'a frightening thing' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'a bad smell doctor' is the wordplay.
'bad smell' becomes 'bo' (body odour).
'doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
'a'+'bo'+'mb'='A-BOMB'
Can you help me to learn more?
(Other definitions for a-bomb that I've seen before include "Weapon of mass destruction" , "deterrent" , "Killer" , "Nuclear weapon" , "explosive" .)