Arming the riotous is a horrifying experience (9)
I believe the answer is:
nightmare
'a horrifying experience' is the definition.
(I've seen this before)
'arming the riotous' is the wordplay.
'riotous' is an anagram indicator.
'arming'+'the'='armingthe'
'armingthe' is an anagram of 'NIGHTMARE'.
'is' acts as a link.
(Other definitions for nightmare that I've seen before include "Scary nocturnal fantasy" , "Very unpleasant experience" , "Frightening nocturnal fantasy" , "that might upset the rest" , "It makes one restless" .)