Write about melody part of Holst's famed suite (7)
I believe the answer is:
neptune
'part of holst's famed suite' is the definition.
(Holst wrote The Planets)
'write about melody' is the wordplay.
'write' becomes 'pen' (synonyms).
'about' is a reversal indicator.
'melody' becomes 'tune' (synonyms).
'pen' reversed gives 'nep'.
'nep'+'tune'='NEPTUNE'
(Other definitions for neptune that I've seen before include "World" , "King of the sea" , "part of Planets Suite" , "Holst movement?" , "One distanced from The Sun" .)