Old rioter in trouble outside (8)
I believe the answer is:
exterior
'outside' is the definition.
(synonyms)
'old rioter in trouble' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'in trouble' indicates anagramming the letters.
'rioter' is an anagram of 'terior'.
'ex'+'terior'='EXTERIOR'
(Other definitions for exterior that I've seen before include "Outside" , "skin" , "Outer surface" , "Facade" , "covering" .)