As Saturnalia is to a Roman in prime (7)
I believe the answer is:
festive
'as saturnalia' is the definition.
(Saturnalia was a Roman festival)
'is to a roman in prime' is the wordplay.
'is to a roman' becomes 'est' ('is' in Latin).
'in' is an insertion indicator.
'prime' becomes 'five' (prime number).
'est' inserted within 'five' is 'FESTIVE'.
(Other definitions for festive that I've seen before include "Celebratory, joyful" , "so celebratory an occasion" , "Offering fun and gaiety" , "Gay" , "Merry" .)