In a mood that's low (3)

I believe the answer is:

moo

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

I don't understand the remainder of the clue.

(Other definitions for moo that I've seen before include "Cow's sound" , "Bovine low" , "Guernsey noise?" , "Farmyard sound" , "Cow's call" .)

I've seen this clue in the Evening Standard.

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