Clark and Mellon worked together at Barnsley and the 20-year-old midfielder has said that after a successful trial period, he is grateful for the opportunity and can't wait to get started.
"Without him being here I don't think I would maybe have got the chance to come here," he said to the club's official website. "I've got to be really thankful for him so I'll thank him later when I see him!
"I know Micky and I know what he wants, how he wants to play so I know what he's thinking. I know if you play and do what he wants then you'll be successful and I think the club will be successful this year."
He added: "It's been a great two-and-a-half weeks, three weeks, coming in as a trialist. The lads and the manager have made me feel really welcome so I couldn't have asked for any more than that.
"Now I'm just looking forward to kicking on and looking forward to the season starting."