Yin's opposite (4)
I believe the answer is:
'yin's opposite' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for yang that I've seen before include "Positive, bright and masculine principle of Chinese philosophy" , "Chinese male principle" , "positive force" , "Positive element in traditional Chinese philosophy" , "Yin counterpart" .)