Write about air on a planet (7)
I believe the answer is:
neptune
'planet' is the definition.
(Neptune is an example)
'write about air' is the wordplay.
'write' becomes 'pen' (synonyms).
'about' is a reversal indicator.
'air' becomes 'tune' (synonyms).
'pen' reversed gives 'nep'.
'nep'+'tune'='NEPTUNE'
'on a' is the link.
(Other definitions for neptune that I've seen before include "Eighth rock from the Sun" , "Holst movement?" , "World" , "musical work" , "Poseidon" .)