Troubled king coherent, though losing head (4)
I believe the answer is:
lear
'troubled king' is the definition.
(I've seen this before)
'losing head' is the wordplay.
'losing' becomes 'l' (this might be a standard abbreviation which I don't know about).
'head' becomes 'ear' (I have seen 'Head of corn' mean 'ear' so perhaps 'head' could also mean 'ear').
'l'+'ear'='LEAR'
'coherent though' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for lear that I've seen before include "Tragic king in play" , "Lesson" , "Fictional king - English humorist" , "Mad king" , "hardly the 7 [NO NONSENSE] type!" .)