Orchestra leader to write tune out of doors (4-3)
I believe the answer is:
open-air
'out of doors' is the definition.
(I've seen this before)
'orchestra leader to write tune' is the wordplay.
'leader' indicates taking the first letters.
'to' is a charade indicator (letters next to each other).
'write' becomes 'pen' (synonyms).
'tune' becomes 'air' (air can mean a piece of music).
The first letter of 'orchestra' is 'o'.
'o'+'pen'+'air'='OPEN-AIR'
(Other definitions for open-air that I've seen before include "Exposed to the elements" , "outside" , "Alfresco" , "Taking place outdoors" , "not below roof" .)