Being low, it obviously isn't a full moon (3)

I believe the answer is:

moo

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'being low' is the definition.
(both can mean to make the noise of a cow)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for moo that I've seen before include "unpopular woman" , "Low (of cattle)" , "Farmyard sound" , "Cow's call" , "Guernsey noise?" .)

I've seen this clue in the Evening Standard.

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