Succeeded opposing work on area for capital city (8)
I believe the answer is:
'capital city' is the definition.
(Santiago is the capital of Chile)
'succeeded opposing work on area' is the wordplay.
'succeeded' becomes 's' (genealogical abbreviation).
'opposing' becomes 'anti' (prefix meaning 'opposing').
'work' becomes 'go' (synonyms).
'on' is a charade indicator (letters next to each other).
'area' becomes 'a' (maths abbreviation).
'go' put after 'a' is 'ago'.
'for' acts as a link.
(Other definitions for santiago that I've seen before include "Chile's capital" , "Chilean capital" , "South American capital" , "Capital of Chile" , "City" .)