Capital forfeits one seat (4)
I believe the answer is:
sofa
'seat' is the definition.
(sofa is a kind of seat)
'capital forfeits one' is the wordplay.
'capital' becomes 'sofia' (Sofia is an example).
'forfeits' suggests deleting specific letters.
'one' becomes 'i' (Roman numeral).
'sofia' with 'i' taken out is 'SOFA'.
(Other definitions for sofa that I've seen before include "Divan" , "Chesterfield, maybe" , "Settee" , "Upholstered seat for two or more" , "Wide padded seat" .)