Beginning working group (5)
I believe the answer is:
onset
'beginning' is the definition.
(onset is a kind of beginning)
'working group' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'group' becomes 'set' (both can mean a collection).
'on'+'set'='ONSET'
(Other definitions for onset that I've seen before include "Cast stone in attack" , "Beginning; violent attack" , "Inception" , "Storming" , "Start - opening moments" .)