Crossword Genius

A god to swear by? (7)

Ross

I believe the answer is:

jupiter

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'a god' is the definition.
(Jupiter is an example)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for jupiter that I've seen before include "to which Tom Towers contributed" , "Planet - Roman equivalent of Zeus" , "Largest planet in our solar system" , "Chief of the Gods, by Jove" , "Heavenly body" .)

I've seen this clue in the Evening Standard.
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