Old writer finds tune outside (4,3)
I believe the answer is:
open air
'outside' is the definition.
(open air is a kind of outside)
'old writer finds tune' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'writer' becomes 'pen' (something which writes).
'finds' is a charade indicator (letters next to each other).
'tune' becomes 'air' (air can mean a piece of music).
'o'+'pen'+'air'='OPEN-AIR'
(Other definitions for open air that I've seen before include "Out of doors" , "natural setting" , "Outdoor" , "theatre" , "Unenclosed space outdoors" .)