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