On the house that's vacant (4)
I believe the answer is:
free
I believe this is a double definition.
'on the house' is the first definition.
(something on the house is given away free)
'vacant' is the second definition.
(thesaurus)
'that's' acts as a link.
(Other definitions for free that I've seen before include "Liberated" , "Not under restraint" , "Release; gratis" , "Complimentary" , "unemployed" .)