Established in a number of celebrations (7)
I believe the answer is:
festive
'of celebrations' is the definition.
(festive means relating to celebrations)
'established in a number' is the wordplay.
'established' becomes 'est' (abbreviated used in shop signs).
'in' means one lot of letters goes inside another.
'a number' becomes 'five' (five is a number).
'est' inserted inside 'five' is 'FESTIVE'.
(Other definitions for festive that I've seen before include "Celebratory, joyful" , "Jolly" , "Gay" , "Offering fun and gaiety" , "so celebratory an occasion" .)