Start performing with group (5)
I believe the answer is:
onset
'start' is the definition.
(onset is a kind of start)
'performing with group' is the wordplay.
'performing' becomes 'on' (I've seen this in other clues).
'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 "Inception" , "Initial stage; attack" , "starting?" , "Start, beginning" , "Cast stone in attack" .)