Table schema:
CREATE TABLE table_name_93 (opponent VARCHAR, date VARCHAR, championship VARCHAR, surface VARCHAR, outcome VARCHAR)

Question: 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"