A good but not high level of illumination (4)
I believe the answer is:
glow
'level of illumination' is the definition.
The answer and definition can be both human attributes as well as being singular nouns.
Maybe there's an association between them I don't understand?
'a good but not high' is the wordplay.
'a good but' becomes 'g' (I can't explain this - if you can you should give a lot more credence to this answer).
'not high' becomes 'low' ('low' is the opposite to 'high').
'g'+'low'='GLOW'
Can you help me to learn more?
(Other definitions for glow that I've seen before include "Give out light without flame" , "Radiate or redden" , "Emanate light" , "Gentle, warm light" , "rosy colour" .)