Being low, it obviously isn't a full moon (3)
I believe the answer is:
moo
'being low' is the definition.
(both can mean to make the noise of a cow)
I can't explain the rest of the clue.
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?" .)