The convivial quintet set off in it (7)
I believe the answer is:
festive
'the convivial' is the definition.
(thesaurus)
'quintet set off in it' is the wordplay.
'quintet' becomes 'five' (synonyms).
'off' indicates an anagram.
'in it' means one lot of letters goes inside another.
'set' is an anagram of 'est'.
'five' going around 'est' is 'FESTIVE'.
(Other definitions for festive that I've seen before include "Celebratory, joyful" , "Merry" , "Jolly" , "so celebratory an occasion" , "Gay" .)