Start working with group (5)
I believe the answer is:
onset
'start' is the definition.
(both can mean the beginning of something)
'working with group' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'with' says to put letters next to each other.
'group' becomes 'set' (both can mean a collection).
'on'+'set'='ONSET'
(Other definitions for onset that I've seen before include "Initial stage; attack" , "Start - attack" , "Cast stone in attack" , "starting?" , "Outbreak" .)