Chambers gridlocked by mafia trials (5)
I believe the answer is:
atria
'chambers' is the definition.
(atrium is a kind of chamber)
'gridlocked by mafia trials' is the wordplay.
'gridlocked by' indicates a hidden word.
'ATRIA' is hidden within 'mafia trials'.
(Other definitions for atria that I've seen before include "Central courts, halls" , "Apartments" , "Lobbies" , "Public spaces with galleries" , "entrances" .)