Simplicity of each essentially delusional head (8)
I believe the answer is:
easiness
'simplicity' is the definition.
(synonyms)
'each essentially delusional head' is the wordplay.
'each' becomes 'ea' (abbreviation).
'essentially' means to look at the middle letters.
'head' becomes 'ness' (ness is a headland).
The centre of 'delusional' is 'si'.
'ea'+'si'+'ness'='EASINESS'
'of' is the link.
(Other definitions for easiness that I've seen before include "Lack of difficulty" , "Nonchalance" , "Facile quality" .)