Opening, working with group (5)
I believe the answer is:
onset
'opening' is the definition.
(I've seen this before)
'working with group' is the wordplay.
'working' becomes 'on'.
'with' means one lot of letters go next to another.
'group' becomes 'set' (set is a kind of group).
'on'+'set'='ONSET'
(Other definitions for onset that I've seen before include "Beginning or early stages" , "Advent" , "Start - opening moments" , "Start - attack" , "Cast stone in attack" .)