Did the ancient Romans swear by him? (7)

I believe the answer is:

jupiter

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'did the ancient romans swear by him?' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.

This is the entire clue.

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(Other definitions for jupiter that I've seen before include "Fifth planet from sun" , "celestial body" , "Chief Roman god" , "Large planet" , "Heavenly body" .)

I've seen this clue in the Evening Standard.

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