Capital piece of furniture envelops one (5)
I believe the answer is:
sofia
'capital' is the definition.
(capital of Bulgaria)
'piece of furniture envelops one' is the wordplay.
'piece of furniture' becomes 'sofa' (sofa is a type of furniture).
'envelops' means one lot of letters goes inside another (I've seen this in other clues).
'one' becomes 'i' (Roman numeral).
'sofa' going around 'i' is 'SOFIA'.
(Other definitions for sofia that I've seen before include "Foreign city" , "Balkan capital" , "European city" , "seat of government" , "capital place" .)