Landlords should be locked up (6,4)
I believe the answer is:
behind bars
I believe this clue is a double definition.
'landlords should be' is the first definition.
(I've seen this before)
'locked up' is the second definition.
(I've seen this before)
(Other definitions for behind bars that I've seen before include "Responsible for some music" , "being detained?" , "Where alcohol is kept" , "In jail" , "Where to find convicts" .)