Bum stops in prison (6,4)
I believe the answer is:
'in prison' is the definition.
(behind bars means locked away in prison)
'bum stops' is the wordplay.
'bum' becomes 'behind' (synonyms).
'stops' becomes 'bars' (bar can mean to stop or prevent).
(Other definitions for behind bars that I've seen before include "Where landlords might be" , "inside" , "Locked away" , "not being free" , "Incarcerated" .)