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' means one lot of letters go next to another.
'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" .)