Outside ring, interrogate a thug (7)
I believe the answer is:
gorilla
'thug' is the definition.
(thesaurus)
'outside ring interrogate a' is the wordplay.
'outside' is an insertion indicator.
'ring' becomes 'o'.
'interrogate' becomes 'grill' (I've seen this before).
'o' put within 'grill' is 'gorill'.
'gorill'+'a'='GORILLA'
(Other definitions for gorilla that I've seen before include "Large animal" , "huge beast" , "Large chest-beating ape" , "Ape - aggressive muscleman" , "Chest-beating primate" .)