The talented 16-year-old midfielder, an England youth international, this week joined Liverpool amid interest from Manchester United and Leeds United.
Clark, the son of former United midfielder Lee, said on Instagram: “Thank you to all the staff and my teammates at @nufc for an incredible 7 years and everything you’ve done to guide me to where I am today.
"You’ll always have a special place in my heart.”
Clark, hailed by Bruce as one of the club’s brightest prospects two years ago, became unsettled at the club’s academy last season. He was offered a scholarship at Newcastle, but decided a move to Liverpool was best for his career this summer. The switch was finalised this week.
Bruce feels that the system for such transfers is “flawed”, though he admitted that Newcastle benefited from it when they signed highly-rated midfielder Joe White, who had been with Carlisle United.
Asked about Clark’s move to Liverpool, United’s head coach said: “First and foremost, it’s what the boy and his family wanted. We’re disappointed that we lost him, because nobody wants to lose a good player.
“I do believe in that situation that the system, for me, is flawed. We benefited from the kid we signed from Carlisle, Joe White. Yeah, disappointed, of course we are. I think it’s flawed. He’s been here since he was a boy.
"I think, in the end, it was best for all parties to part company.”