Yin counterpart (4)
I believe the answer is:
yang
'yin counterpart' 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 "Yin's counterpart" , "Complement of yin" , "positive force" , "Yin counterpart" , "Positive element in traditional Chinese philosophy" .)