Doctor said see Head of Special Illnesses (8)
I believe the answer is:
diseases
'illnesses' is the definition.
(disease is a kind of illness)
'doctor said see head of special' is the wordplay.
'doctor' is an anagram indicator.
'head of' indicates taking the first letters.
The initial letter of 'special' is 's'.
'said'+'see'='saidsee'
'saidsee' is an anagram of 'disease'.
'disease'+'s'='DISEASES'
(Other definitions for diseases that I've seen before include "Afflictions, disorders" , "Sicknesses" , "Illnesses, maladies" , "Disorders, illnesses" , "Illness brought on by infections or diet" .)