Dreadful rule referring to riotous soldiers (5,2,6)
I believe the answer is:
reign of terror
'dreadful rule' is the definition.
(I know this)
'referring to riotous soldiers' is the wordplay.
'riotous' indicates an anagram.
'soldiers' becomes 'OR' (Other Ranks).
'referring'+'to'='referringto'
'referringto' with letters rearranged gives 'reignofterr'.
'reignofterr'+'or'='REIGN OF TERROR'
(Other definitions for reign of terror that I've seen before include "awful time in France" , "French Revolutionary experience" , "Bloody period of French Revolution" , "Period of Jacobin rule in France" , "period of brutal oppression" .)