Ratty, one criminal lawyer (8)
I believe the answer is:
attorney
'lawyer' is the definition.
('attorney' can be a synonym of 'lawyer')
'ratty one criminal' is the wordplay.
'criminal' indicates an anagram.
'ratty'+'one'='rattyone'
'rattyone' with letters rearranged gives 'ATTORNEY'.
(Other definitions for attorney that I've seen before include "Legal advisor" , "Lawman" , "- jar; - tent; - metal" , "One legally acting for another" , "Any torte (anag.)" .)