One writing about air in planet (7)
I believe the answer is:
neptune
'planet' is the definition.
(I know that Neptune is an Jovian planet)
'one writing about air' is the wordplay.
'one writing' becomes 'pen'.
'about' shows that the letters should be reversed in order.
'air' becomes 'tune' (synonyms).
'pen' in reverse letter order is 'nep'.
'nep'+'tune'='NEPTUNE'
'in' is the link.
(Other definitions for neptune that I've seen before include "revolutionary body" , "Roman sea-god" , "Roman god of the sea, with trident" , "World" , "Poseidon" .)