Yin's partner (4)
I believe the answer is:
yang
'yin's partner' 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" , "Yin counterpart" , "Positive, bright and masculine principle of Chinese philosophy" , "Complement of yin" , "Chinese male principle" .)