Writer returns by air or sea (7)
I believe the answer is:
neptune
'sea' is the definition.
The definition and answer can be both natural objects as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'writer returns by air' is the wordplay.
'writer' becomes 'pen' (something which writes).
'returns' says the letters should be written in reverse.
'by' is a charade indicator (letters next to each other).
'air' becomes 'tune' (air can mean a piece of music).
'pen' in reverse letter order is 'nep'.
'nep'+'tune'='NEPTUNE'
'or' acts as a link.
Can you help me to learn more?
(Other definitions for neptune that I've seen before include "part of Planets Suite" , "musical work" , "heavenly body" , "Roman sea-god" , "extract from The Planets" .)