On which one needn't sit alone (4)
I believe the answer is:
sofa
If this is the answer I cannot explain how the clue works.
Can you help me to learn more?
(Other definitions for sofa that I've seen before include "Comfy piece of furniture" , "Chesterfield, maybe" , "Upholstered seat for more than one person" , "Upholstered seating" , "Living room seat" .)