Start rolling stone (5)
I believe the answer is:
onset
'start' is the definition.
(both can mean the beginning of something)
'rolling stone' is the wordplay.
'rolling' indicates anagramming the letters.
'stone' is an anagram of 'ONSET'.
(Other definitions for onset that I've seen before include "starting?" , "Beginning (of something often unpleasant)" , "17 down [ASSAULT]" , "Start, beginning" , "Beginning or early stages" .)