Yang's opposite (3)
I believe the answer is:
'yang's opposite' 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" , "Chinese female passive principle of universe" , "Complement of yang" , "principle accepted in China" , "Comedian Billy Connolly is the big one" .)