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