Mention a misapprehension that head has left (8)
I believe the answer is:
allusion
'mention' is the definition.
(I know that allusion is a type of mention)
'a misapprehension that head has left' is the wordplay.
'misapprehension' becomes 'illusion' (believing an illusion is misapprehending it).
'that head has left' means to remove the first letter.
'illusion' with its initial letter removed is 'llusion'.
'a'+'llusion'='ALLUSION'
(Other definitions for allusion that I've seen before include "Passing reference or indirect mention" , "Subtle reference" , "A brief or indirect reference to something" .)