Write ascending melody for sea god (7)
I believe the answer is:
neptune
'sea god' is the definition.
(thesaurus)
'write ascending melody' is the wordplay.
'write' becomes 'pen' ('pen' can be a synonym of 'write').
'ascending' is a reversal indicator.
'melody' becomes 'tune' ('tune' can be a synonym of 'melody').
'pen' written backwards gives 'nep'.
'nep'+'tune'='NEPTUNE'
'for' is the link.
(Other definitions for neptune that I've seen before include "Tune pen into the Roman god" , "Roman sea-god" , "in Holst's suite" , "one of the Planets" , "Roman god of the sea, with trident" .)