Former partner with unruly rioter outside (8)
I believe the answer is:
exterior
'outside' is the definition.
(similar in meaning)
'former partner with unruly rioter' is the wordplay.
'former partner' becomes 'ex' (an ex-partner).
'with' means one lot of letters go next to another.
'unruly' indicates anagramming the letters.
'rioter' is an anagram of 'terior'.
'ex'+'terior'='EXTERIOR'
(Other definitions for exterior that I've seen before include "Retire ox (anag)" , "Face" , "covering" , "Foreign" , "skin" .)