The main deity's heavenly body (7)

I believe the answer is:

neptune

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This could be a double definition.

'the main deity's' is the first definition.
'Neptune' can be an answer for 'deity's' (Neptune is an example). I am unsure of the 'the main' bit.

'heavenly body' is the second definition.
(Neptune is an example)

Can you help me to learn more?

(Other definitions for neptune that I've seen before include "one of the Planets" , "Roman god of water and the sea" , "musical work" , "revolutionary body" , "Roman sea-god" .)

I've seen this clue in The Sun.

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