Begin song outdoors (4,3)
I believe the answer is:
open air
'outdoors' is the definition.
('open air' can be a synonym of 'outdoors')
'begin song' is the wordplay.
'begin' becomes 'open' (opening is a kind of begining).
'song' becomes 'air' (I've seen this before).
'open'+'air'='OPEN-AIR'
(Other definitions for open air that I've seen before include "Unenclosed space outdoors" , "natural setting" , "Outside" , "theatre" , "concerts?" .)