According to legend, people originally lived outside Rome in catacombs (9)
I believe the answer is:
folkloric
'according to legend' is the definition.
I can't judge whether this defines the answer.
'people originally lived outside rome in catacombs' is the wordplay.
'people' becomes 'folk' (informal term for people).
'originally' indicates taking the first letters.
The first letters of 'lived outside rome in catacombs' is 'loric'.
'folk'+'loric'='FOLKLORIC'
Can you help me to learn more?
(Another definition for folkloric that I've seen is " As legend would have it".)