It was the Baggies' first win at Old Trafford since 1978 and the result will come as a huge shock to a team that won the title by 11 points last term.
But Clarke said: "Anybody listening to me before the game would know I said I was coming here to be positive to try to play.
"The players must have been listening as well because they did that.
"Against the reigning champions you have to play almost the perfect game.
"We could have had more. We looked a threat every time we went forward.
"It's not about the fear factor. West Brom has always been capable of coming to these sorts of places, defend doggedly and nick a winner or a draw.
"But I want to try to win the game and we achieved that."
Clarke had particular words of praise for his goalscorers, including Amalfitano, a deadline-day loan signing from Marseille.
"His English is OK," said Clarke. "He understands more than he lets on. When you tell him he's done well he always understands.
"He's a player that I'd seen play on a number of occasions, bright lively and has the flair to create going forward."
But 20-year-old Berahino was the hero, keeping his cool to beat David De Gea from Amalfitano's pass.
"He deserves his place in the team," said Clarke of the England Under-21 international.
"He's made an impact and as you've seen he's got a goal in him."