Writer turned up with simple air — Holst wrote it (7)
I believe the answer is:
neptune
'holst wrote it' is the definition.
(a movement of Gustav Holst's suite The Planets)
'writer turned up with simple air' is the wordplay.
'writer' becomes 'pen' (something which writes).
'turned up' says the letters should be written backwards.
'with' says to put letters next to each other.
'simple air' becomes 'tune' (air can mean a piece of music).
'pen' backwards is 'nep'.
'nep'+'tune'='NEPTUNE'
(Other definitions for neptune that I've seen before include "Holst movement?" , "Planet, identified 1846" , "King of the sea" , "Roman sea god - planet" , "one of the Planets" .)