Feature shared by Seoul, Sofia and Santiago, aptly (8)
I believe the answer is:
capitals
If this is the answer I cannot explain how the clue works.
Can you help me to learn more?
(Other definitions for capitals that I've seen before include "Upper-case letters" , "First letters and first cities" , "First letters, of names say" , "Major cities or large letters" , "E.g. PARIS, ROME" .)