Americans imprison very loud Republican leading Yankee riot (6)
I believe the answer is:
affray
'riot' is the definition.
(thesaurus)
'americans imprison very loud republican leading yankee' is the wordplay.
'americans' becomes 'AA' (two abbreviations for American).
'imprison' means one lot of letters goes inside another.
'very loud' becomes 'ff' (musical abbreviation).
'leading' indicates taking the first letters.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
The initial letter of 'republican' is 'r'.
'ff'+'r'='ffr'
'aa' placed around 'ffr' is 'affra'.
'affra'+'y'='AFFRAY'
(Other definitions for affray that I've seen before include "public brawl" , "Noisy quarrel" , "commotion" , "Breach of the peace by public rioting" , "Brawling" .)