God, write about air (7)
I believe the answer is:
neptune
'god' is the definition.
(Neptune is an example)
'write about air' is the wordplay.
'write' becomes 'pen' (synonyms).
'about' shows that the letters should be reversed in order.
'air' becomes 'tune' (synonyms).
'pen' written backwards gives 'nep'.
'nep'+'tune'='NEPTUNE'
(Other definitions for neptune that I've seen before include "musical work" , "part of Planets Suite" , "one of the Planets" , "King of the sea" , "Holst composed it" .)