Principle behind yin and yang (3)
I believe the answer is:
tao
'principle behind yin and yang' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tao that I've seen before include "true path" , "Element of Chinese philosophy" , "Chinese way to be followed" , "Source of existence (Chinese)" , "The way of Chinese philosophy" .)