Relative supporting estate, say, with billions for architectural eyesore (9)
I believe the answer is:
carbuncle
'architectural eyesore' is the definition.
(carbuncle can mean an ugly building)
'relative supporting estate say with billions' is the wordplay.
'relative' becomes 'uncle' (uncle is a kind of relative).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'estate say' becomes 'car' (I've seen this in other clues).
'with' is a charade indicator (letters next to each other).
'billions' becomes 'b' (abbreviation).
'car'+'b'='carb'
'uncle' after 'carb' is 'CARBUNCLE'.
'for' acts as a link.
(Other definitions for carbuncle that I've seen before include "cheaper jewel?" , "National Gallery's Sainsbury Wing?" , "stone" , "Large boil-like inflammation - gemstone" , "growth" .)