… confound American criminal with Roman law (7)
I believe the answer is:
perplex
'confound' is the definition.
(synonyms)
'american criminal with roman law' is the wordplay.
'american criminal' becomes 'perp' (US term short for perpetrator).
'with' means one lot of letters go next to another.
'roman law' becomes 'lex' ('law' in Latin).
'perp'+'lex'='PERPLEX'
(Other definitions for perplex that I've seen before include "Make (someone) baffled" , "Dumbfound" , "Baffle, puzzle" , "Bamboozle" , "Baffle, flummox" .)