Troubled king coherent, losing head (4)
I believe the answer is:
lear
'troubled king' is the definition.
(Shakespeare's King Lear)
I don't understand how the remainder of the clue works.
(Other definitions for lear that I've seen before include "Nonsense poet; drama king" , "Tragic Shakespearean king" , "Lesson" , "British king in play" , "Tragic Shakespearian king" .)