Start running group (5)
I believe the answer is:
onset
'start' is the definition.
(both can mean the beginning of something)
'running group' is the wordplay.
'running' becomes 'on' (eg when a machine is running it is on).
'group' becomes 'set' (both can mean a collection).
'on'+'set'='ONSET'
(Other definitions for onset that I've seen before include "starting?" , "Start - opening moments" , "Beginning; violent attack" , "Advent" , "Beginning (of something often unpleasant)" .)