Ring round head appearing in encephalogram (4)
I believe the answer is:
halo
'ring round head' is the definition.
(glowing ring around the head of a holy person)
'in encephalogram' is the wordplay.
'in' indicates a hidden word.
'HALO' is hidden in the letters of 'encephalogram'.
'appearing' acts as a link.
(Other definitions for halo that I've seen before include "Light around sun, saint" , "Ring of light (signifying holiness)" , "Saintly ring" , "Ring (around sun or moon)" , "Holy ring" .)