Start throwing stone (5)
I believe the answer is:
onset
'start' is the definition.
(both can mean the beginning of something)
'throwing stone' is the wordplay.
'throwing' indicates an anagram (letters get thrown around).
'stone' is an anagram of 'ONSET'.
(Other definitions for onset that I've seen before include "Beginning; violent attack" , "Appearance of" , "Beginning (of something often unpleasant)" , "Kick-off" , "First signs" .)