Capital one invested in Chesterfield? (5)
I believe the answer is:
sofia
'capital' is the definition.
(capital of Bulgaria)
'one invested in chesterfield?' is the wordplay.
'one' becomes 'i' (Roman numeral).
'invested in' means one lot of letters goes inside another.
'chesterfield?' becomes 'sofa' (chesterfield is a kind of sofa).
'i' put within 'sofa' is 'SOFIA'.
(Other definitions for sofia that I've seen before include "Balkan capital" , "capital place" , "Foreign city" , "Bulgarian city" , "capital city" .)