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