Write backing melody for final movement of suite (7)
I believe the answer is:
neptune
'final movement of suite' is the definition.
(final movement of Gustav Holst's 'The Planets')
'write backing melody' is the wordplay.
'write' becomes 'pen' ('pen' can be a synonym of 'write').
'backing' says the letters should be written in reverse.
'melody' becomes 'tune' ('tune' can be a synonym of 'melody').
'pen' reversed gives 'nep'.
'nep'+'tune'='NEPTUNE'
'for' is the link.
(Other definitions for neptune that I've seen before include "King of the sea" , "One distanced from The Sun" , "Sea god - outer planet" , "musical work" , "Tune pen into the Roman god" .)