Oxygen dismal in air (5)
I believe the answer is:
odour
'air' is the definition.
The answer and definition can be both human attributes as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'oxygen dismal' is the wordplay.
'oxygen' becomes 'O' (O is the chemical symbol for oxygen).
'dismal' becomes 'dour' (I can't explain this - if you can you should believe this answer much more).
'o'+'dour'='ODOUR'
'in' is the link.
Can you help me to learn more?
(Other definitions for odour that I've seen before include "pervasive quality" , "Smell, scent" , "Regard" , "Small, aroma" , "Whiff" .)