Stone shaped in beginning (5)
I believe the answer is:
'beginning' is the definition.
(onset is a kind of beginning)
'stone shaped' is the wordplay.
'shaped' is an anagram indicator.
'stone' with letters rearranged gives 'ONSET'.
'in' is the link.
(Other definitions for onset that I've seen before include "Kick-off" , "Beginning (of something often unpleasant)" , "Storming" , "Start - opening moments" , "Assault; start" .)