Writer comes up with composition for god (7)
I believe the answer is:
neptune
'for god' is the definition.
(Neptune is an example)
'writer comes up with composition' is the wordplay.
'writer' becomes 'pen' (something which writes).
'comes up' says the letters should be written backwards (in a down clue, letters go up).
'with' is a charade indicator (letters next to each other).
'composition' becomes 'tune' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pen' reversed gives 'nep'.
'nep'+'tune'='NEPTUNE'
Can you help me to learn more?
(Other definitions for neptune that I've seen before include "part of Planets Suite" , "Sea god - outer planet" , "in Holst's suite" , "Tune pen into the Roman god" , "Roman sea god - planet" .)