It's very well known to have had a festival (10)
I believe the answer is:
celebrated
'festival' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I can't see?
'it's very well known to have had a' is the wordplay.
I cannot really see how this works, but
'had' could be 'd' (I've seen this in other clues) and 'd' is found in the answer.
'a' is found in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for celebrated that I've seen before include "Did something enjoyable (to mark an event)" , "Notable" , "Cheered" , "Observed (festival); famous" , "Rejoiced - famous" .)