This is certainly still very winnable for Real Madrid. I personally don't believe Real Madrid is going to lose just because they have a disadvantage of 1 goal. Real Madrid have a good experience of making comebacks in the second leg. We have seen that multiple times, especially in the Champions League. Real Madrid already knows that they don't have much chance of winning the La Liga title. So they will shortly try to win the Champions League and the Copa del Rey for sure.
Overall, Barcelona excels in H2H vs Real Madrid this season, Barcelona has successfully won three of the four El Clasico this season. Ahead of this match begins, Barcelona has two advantages. First, Xavi Hernandez team has a capital of 1 goal which they won in the first leg. secondly, this match will be played at the Barcelona stadium. Acting as the host made the Barcelona players more confident, especially in the last El Clasico match in La Liga, Barcelona managed to dominate their might over Real Madrid.
Real Madrid still has a chance to turn things around, they must be able to dominate the game and dismantle Barcelona defense to score goals. Real Madrid who have lost their chance to win La Liga, will fight it out to reach the CDR final, even though their struggle this time is very tough, they have to try even if the final result makes them disappointed.