Intense sorrow of King George that is beginning to fade (5)
I believe the answer is:
grief
'intense sorrow' is the definition.
(I've seen this before)
'king george that is beginning to fade' is the wordplay.
'king george' becomes 'gr' (Georgius Rex).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'beginning to' suggests taking the first letters.
The first letter of 'fade' is 'f'.
'gr'+'ie'+'f'='GRIEF'
'of' is the link.
(Other definitions for grief that I've seen before include "Great sorrow at someone's death" , "Great mourning" , "Intense sorrow caused by loss of a loved one" , "Misery" , "Deep sadness" .)