Lear's affliction created mostly by head (7)
I believe the answer is:
madness
'lear's affliction' is the definition.
I can't tell whether this definition defines the answer.
'created mostly by head' is the wordplay.
'created' becomes 'made' (making is a kind of creating).
'mostly' means to remove the last letter (most but not all of the word).
'by head' becomes 'ness' (ness is a headland. I am not sure about the 'by' bit.).
'made' with its last letter removed is 'mad'.
'mad'+'ness'='MADNESS'
Can you help me to learn more?
(Other definitions for madness that I've seen before include "Folly" , "Utter foolishness" , "hysteria?" , "Dementia" , "Lunacy - ska/pop band" .)