Unthinking king — born idiot! (4-4)
I believe the answer is:
knee jerk
'unthinking' is the definition.
(I've seen this in another clue)
'king born idiot' is the wordplay.
'king' becomes 'K' (abbreviation for king in chess).
'born' becomes 'nee' ('nee' is 'born' in French).
'idiot' becomes 'jerk' (I've seen this in another clue).
'k'+'nee'+'jerk'='KNEE-JERK'
(Other definitions for knee jerk that I've seen before include "Sudden and ill-considered" , "(Of a reaction) automatic and unthinking" , "instinctive" , "Type of automatic reaction" , "Such a predictable reaction" .)