The 21-year-old is a versatile forward and Johnson believes he is a player who has made significant strides over recent months.
“Jack is a talented player and having followed his progress over the past few years, I feel like he’s really grown on the pitch throughout the past six months and found a maturity to pair with that talent,” Johnson said.
“The competition for places in our final third is strong and we want to be able to change games as we move into the back end of the season.
"Jack understands that this is a big club with a lot to play for, but he wants to be an influential player for us and help to ensure we have a successful end to the season.”
Clarke, who broke through at Leeds United before making a £10,000,000 move to Spurs, said: “I can't wait to get started. I'm from this part of the world, so I know this is a massive club and what it means to the fans.
“I'm looking forward to playing at the Stadium of Light and the prospect of being part of the team that takes the club back to the Championship. I want to help the team as much as possible, whether that's goals or assists, and ensure we reach that end goal."