Very distressing war never troubled China's foremost ruler (5-8)
I believe the answer is:
nerve-wracking
'very distressing' is the definition.
(nerve-wracking means very worrying)
'war never troubled china's foremost ruler' is the wordplay.
'troubled' indicates anagramming the letters.
'foremost' indicates taking the first letters.
'ruler' becomes 'king' (king is a kind of ruler).
The first letter of 'chinas' is 'c'.
'war'+'never'='warnever'
'warnever' anagrammed gives 'nervewra'.
'nervewra'+'c'+'king'='NERVE-WRACKING'
(Other definitions for nerve-wracking that I've seen before include "Causing anxiety" , "Causing anxiety/stress" .)