Celebratory number contains beat but no bass (7)
I believe the answer is:
festive
'celebratory' is the definition.
(I've seen this before)
'number contains beat but no bass' is the wordplay.
'number' becomes 'five' (five is a kind of number).
'contains' indicates putting letters inside.
'beat' becomes 'best' (both can mean to top or outdo).
'but no' suggests deleting specific letters.
'bass' becomes 'b' (musical abbreviation).
'best' with 'b' taken out is 'est'.
'five' placed around 'est' is 'FESTIVE'.
(Other definitions for festive that I've seen before include "Celebratory, joyful" , "Offering fun and gaiety" , "Celebrating" , "Gay" , "Merry" .)