Start working group (5)
I believe the answer is:
onset
'start' is the definition.
(onset is a kind of start)
'working group' is the wordplay.
'working' becomes 'on'.
'group' becomes 'set' (set is a kind of group).
'on'+'set'='ONSET'
(Other definitions for onset that I've seen before include "Start - notes (anag)" , "17 down [ASSAULT]" , "Initial stage; attack" , "Outbreak" , "Storming" .)