Unconscious king once known as fool (4-4)
I believe the answer is:
knee-jerk
'unconscious' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'king once known as fool' is the wordplay.
'king' becomes 'K' (chess abbreviation).
'once known as' becomes 'nee' (I've seen this in another clue).
'fool' becomes 'jerk' (jerk can mean a foolish person).
'k'+'nee'+'jerk'='KNEE-JERK'
Can you help me to learn more?
(Other definitions for knee-jerk that I've seen before include "(Of a reaction) automatic and unthinking" , "Sudden and ill-considered" , "(Of a response) automatic and unthinking" , "Thoughtlessly spontaneous" , "Involuntary or hasty (reaction)" .)