user: Question: who is the opponent when the surface is Clay, the outcome is Winner and the championship is Estoril on 15 April 1996?
Columns: ['Outcome', 'Date', 'Championship', 'Surface', 'Opponent', 'Score']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Winner', '4 August 1986', 'Hilversum', 'Clay', 'Jakob Hlasek', '6–1, 6–3, 6–3']
SQL:

assistant: SELECT "Opponent" FROM "Table" WHERE "Surface" = 'Clay' AND "Outcome" = 'Winner' AND "Championship" = 'Estoril' AND "Date" = '15 April 1996';