Charlton forward Clarke joined the Iron on a three-month loan deal late last week and scored just four minutes into his debut on Saturday in the 1-1 home draw with Sheffield United.
"His wages are way above what we could afford to pay," Knill told the club's official website.
"He's unlucky that he's not starting for Charlton, but that's not because he's playing badly, but that those in front of him are playing well.
"So we said to him to come here and get some games and hopefully some goals. Then he hopefully will get an opportunity to play in the Championship or above. We think he's good enough to do that."