Most troublesome, riots then breaking out (9)
I believe the answer is:
thorniest
'most troublesome' is the definition.
'riots then breaking out' is the wordplay.
'breaking out' indicates an anagram.
'riots'+'then'='riotsthen'
'riotsthen' anagrammed gives 'THORNIEST'.
(Other definitions for thorniest that I've seen before include "most problematic" , "Most troublesome or prickly" .)
I've seen this clue in The Telegraph.
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