Bet I'm late, ruining the schedule! (9)
I believe the answer is:
timetable
'schedule' is the definition.
(timetable is a kind of schedule)
'bet i'm late ruining' is the wordplay.
'ruining' indicates anagramming the letters.
'bet'+'im'+'late'='betimlate'
'betimlate' anagrammed gives 'TIMETABLE'.
'the' acts as a link.
(Other definitions for timetable that I've seen before include "Schedule of events" , "Syllabus" , "Programme" .)