Heads of State highly enthusiastic about religion (6)
I believe the answer is:
shinto
'religion' is the definition.
(Shinto is an example)
'heads of state highly enthusiastic' is the wordplay.
'heads of' indicates taking the first letters.
'enthusiastic' becomes 'into' (I've seen this in other clues).
The first letters of 'state highly' is 'sh'.
'sh'+'into'='SHINTO'
'about' acts as a link.
(Other definitions for shinto that I've seen before include "Religious system" , "Japanese religion revering ancestors" , "Ancestor-worshipping religion of Japan" , "State religion of Japan until 1945" , "Japanese cult" .)