Begin singing the song outside (4-3)
I believe the answer is:
open-air
'outside' is the definition.
(open air is a kind of outside)
'begin singing the song' is the wordplay.
I cannot really see how this works, but
'begin' could be 'open' (both can mean to start) and 'open' is found in the answer.
'song' could be 'air' (air can mean a piece of music) and 'air' is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for open-air that I've seen before include "Exposed to the elements" , "Outdoor" , "outside" , "not below roof" , "Taking place outdoors" .)