Hum nothing gloomy (5)

I believe the answer is:

odour

I'm a little stuck... Click here to teach me more about this clue!

'hum' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

'nothing gloomy' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'gloomy' becomes 'dour' (I've seen this in other clues).
'o'+'dour'='ODOUR'

Can you help me to learn more?

(Other definitions for odour that I've seen before include "Perfume" , "Smell, redolence" , "Distinctive smell" , "Regard" , "Olfactory sensation" .)

I've seen this clue in The Times.

Want a hint initially instead of a full solution? Install my app