It's blue beneath Oz (4,5)
I believe the answer is:
down under
'oz' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?
'it's blue beneath' is the wordplay.
'it's blue' becomes 'down' (both can mean sad).
'beneath' becomes 'under' (under means beneath or underneath).
'down'+'under'='DOWN UNDER'
Can you help me to learn more?
(Other definitions for down under that I've seen before include "Location of those in the Antipodes" , "Far beneath, like Australia" , "Informal location of Australia" , "Informally in the antipodes" , "In Oz, say?" .)