Sofa or chair (4)
I believe the answer is:
seat
I believe this is a double definition.
'sofa' is the first definition.
(thesaurus)
'chair' is the second definition.
(something to sit on)
(Other definitions for seat that I've seen before include "Chair, bench, saddle, etc" , "Chair, bench, or stool, say" , "Parliamentary constituency" , "'A chair, say (4)'" , "Horizontal part of a chair" .)