Start breaking stone (5)
I believe the answer is:
onset
'start' is the definition.
(onset is a kind of start)
'breaking stone' is the wordplay.
'breaking' indicates an anagram.
'stone' anagrammed gives 'ONSET'.
(Other definitions for onset that I've seen before include "Advent" , "origin" , "Assault; start" , "Beginning; violent attack" , "Inception" .)