Presumably the wettest of the planets? (7)

I believe the answer is:

neptune

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'of the planets?' is the definition.
The definition suggests a singular noun which matches the answer.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for neptune that I've seen before include "Holst composed it" , "Roman sea god - planet" , "Roman god of water and the sea" , "heavenly body" , "one of the Planets" .)

I've seen this clue in the Evening Standard.

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