It was frightful as a rule (5,2,6)
I believe the answer is:
reign of terror
'it was frightful' is the definition.
The definition suggests a singular noun which matches the answer.
'a rule' is the wordplay.
I cannot really see how this works, but
'rule' could be 'reign' (reign is a kind of rule) and 'reign' is found within the answer.
This explanation may well be incorrect...
'as' acts as a link.
Can you help me to learn more?
(Other definitions for reign of terror that I've seen before include "period of brutal oppression" , "Period of Jacobin rule in France" , "French Revolutionary experience" , "What Robespierre started" , "awful time in France" .)