Yang's partner (3)
I believe the answer is:
yin
'yang's partner' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for yin that I've seen before include "Principle of cold and dark" , "Complement of yang" , "Yang's counterpart" , "Counterpart of yang" , "Chinese female passive principle of universe" .)