Like a summer event: begin with song (4-3)
I believe the answer is:
open-air
'like a summer event' is the definition.
'begin with song' is the wordplay.
'begin' becomes 'open' (opening is a kind of begining).
'with' means one lot of letters go next to another.
'song' becomes 'air' (I've seen this before).
'open'+'air'='OPEN-AIR'
(Other definitions for open-air that I've seen before include "outside" , "Taking place outdoors" , "Outdoor" , "Alfresco" , "not below roof" .)