Write about air on a planet (7)
I believe the answer is:
neptune
'a planet' is the definition.
(Neptune is an example)
'write about air on' is the wordplay.
'write' becomes 'pen' (synonyms).
'about' is a reversal indicator.
'air' becomes 'tune' (air can mean a piece of music).
'on' says to put letters next to each other.
'pen' backwards is 'nep'.
'nep'+'tune'='NEPTUNE'
(Other definitions for neptune that I've seen before include "Eighth rock from the Sun" , "Holst movement?" , "World" , "musical work" , "Poseidon" .)