Yang partner (3)
I believe the answer is:
yin
'yang partner' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for yin that I've seen before include "Yang's counterpart" , "Chinese female passive principle of universe" , "Counterpart of yang" , "Comedian Billy Connolly is the big one" , "Complement of yang" .)