Clayton spent part of last season on-loan at Carlisle.
Abbott said: "He found his way blocked at Man City, but we gave him a full season of football last year and he was terrific.
"We were obviously chasing his signature (again), but we knew he'd probably go to the Championship and a big club, which he has done at Leeds.
"I think he'll do very well. Once he breaks into the team, Leeds are going to see a really creative, hard-working midfield player.
"He's got loads of ability and has had a bit of a learning curve now with the Man City thing and that will stand him in good stead. He came out to us and found out what the real football world is like and got better and better.
"As a long-term signing for Leeds, I think he could go onto better things and be a really good prospect.
"He's an all-round good player. We tried to work on his side of the game where he hasn't got the ball. But when he has it, he creates things, has a good attitude and makes things happen and wants to score a goal and find a pass. He's really comfortable on the ball.
"Going forward, certainly at League One level, he was very, very good.
"He's also a great lad; a typical young footballer making his way in the game. He's got plenty to say for himself, but in a nice way. We had a really good rapport with him and he's a nice way about him."