Crossword Genius

Neptune's promise to keep left? (5)

Ross

I believe the answer is:

world

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'neptune's' is the definition.
The definition and answer can be both natural objects as well as being singular nouns.
Maybe you can see an association between them that I don't see?

'promise to keep left?' is the wordplay.
'promise' becomes 'word' (as in 'I give you my word').
'to keep' is an insertion indicator.
'left?' becomes 'l' (common abbreviation).
'word' placed around 'l' is 'WORLD'.

Can you help me to learn more?

(Other definitions for world that I've seen before include "we're happy to be on top of" , "Cosmos" , "Planet like the 7 down" , "place Columbus set sail from with 1a [OLD]" , "Everyone" .)

I've seen this clue in The Guardian.
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