Jolly number having set off (7)
I believe the answer is:
festive
'jolly' is the definition.
(I've seen this before)
'number having set off' is the wordplay.
'number' becomes 'five' (five is a kind of whole number).
'having' is an insertion indicator.
'off' is an anagram indicator.
'set' is an anagram of 'est'.
'five' going around 'est' is 'FESTIVE'.
(Other definitions for festive that I've seen before include "Gay" , "Offering fun and gaiety" , "Celebratory, joyful" , "Christmassy" , "Celebrating" .)