A state of somnolence (4,2,3)
I believe the answer is:
land of nod
'a state of somnolence' is the definition.
The definition suggests a singular noun which matches the answer.
'state of somnolence' is the wordplay.
'state' becomes 'land' (synonyms).
'somnolence' becomes 'nod' (I can't explain this - if you can you should believe this answer much more).
'land'+'of'+'nod'='LAND OF NOD'
Can you help me to learn more?
(Other definitions for land of nod that I've seen before include "Imaginary place of sleep" , "Where Cain went after leaving Eden" , "Early dwelling place" , "east of 27 [EDEN]" , "Where Cain fled to, east of Eden" .)