Beginning, start (5)
I believe the answer is:
onset
I believe this clue is a double definition.
'beginning' is the first definition.
(I know that beginning can be written as onset)
'start' is the second definition.
(both can mean the beginning of something)
(Other definitions for onset that I've seen before include "Inception" , "Appearance of" , "Outbreak" , "Beginning or early stages" , "Kick-off" .)