Contrary writer with the air of a Roman god (7)
I believe the answer is:
neptune
'a roman god' is the definition.
(Neptune is an example)
'contrary writer with the air' is the wordplay.
'contrary' shows that the letters should be reversed in order.
'writer with' becomes 'pen' (I've seen this in other clues).
'the air' becomes 'tune' (air can mean a piece of music).
'pen' backwards is 'nep'.
'nep'+'tune'='NEPTUNE'
'of' acts as a link.
(Other definitions for neptune that I've seen before include "Eighth rock from the Sun" , "Planet, identified 1846" , "part of Planets Suite" , "King of the sea" , "heavenly body" .)