Religious festival established as most religious of all (7)
I believe the answer is:
holiest
'most religious of all' is the definition.
I can't judge whether this definition defines the answer.
'religious festival established' is the wordplay.
'religious festival' becomes 'holi' (I've seen this before. I am not sure about the 'religious' bit.).
'established' becomes 'est' (abbreviation).
'holi'+'est'='HOLIEST'
'as' is the link.
Can you help me to learn more?
(Other definitions for holiest that I've seen before include "Most divine" , "most befitting to worship" , "Most pious" , "superlatively untouchable" , "Most resembling sacred work" .)