Celebrate finding one in schedule (7)
I believe the answer is:
roister
'celebrate' is the definition.
(I know that roister is a type of jollify)
'one in schedule' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'schedule' becomes 'roster' (I've seen this before).
'i' put within 'roster' is 'ROISTER'.
'finding' is the link.
(Other definitions for roister that I've seen before include "Be boisterous" , "I rest or (anag) - engage in noisy merrymaking" , "Behave boisterously" , "Have noisy fun" , "Rioters (anag.)" .)