Loss of brightness, with black day between spells of bad weather (5,5)
I believe the answer is:
brain drain
'loss of brightness' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'black day between spells of bad weather' is the wordplay.
'black' becomes 'b' (abbreviation as in pencil classifications).
'day' becomes 'd' (abbreviation for day).
'between' means one lot of letters goes inside another.
'spells of' becomes 'rain' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'bad weather' becomes 'rain' (rain is an example of bad weather).
'rain'+'rain'='rainrain'
'd' going into 'rainrain' is 'raindrain'.
'b'+'raindrain'='BRAIN DRAIN'
'with' is the link.
Can you help me to learn more?
(Other definitions for brain drain that I've seen before include "Emigration of well-educated people" , "Movement of highly skilled people" , "Emigration of talent" , "Haemorrhaging of talent" , "Migration of talent" .)