CREATE TABLE table_name_93 (opponent VARCHAR(255), date VARCHAR(255), championship VARCHAR(255), surface VARCHAR(255), outcome VARCHAR(255))
who is the opponent when the surface is clay, the outcome is winner and the championship is estoril on 15 april 1996?
    
SQL: SELECT opponent FROM table_name_93 WHERE surface = "clay" AND outcome = "winner" AND championship = "estoril" AND date = "15 april 1996"