Three-quarter moon is low (3)
I believe the answer is:
moo
'is low' is the definition.
(both can mean to make a cow noise)
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 "Cartoon sound of a cow" , "unpopular woman" , "Guernsey noise?" , "Irritating woman" , "Low (of cattle)" .)