The 22-year-old was initially named on the bench, having been included in the senior squad in the last few international breaks after a lengthy run as U21s captain.
However, with former Norwich City goalkeeper Angus Gunn pulling up with an injury, Slicker was brought on in the seventh minute for his first appearance. He was supposed to be the third choice, but Kilmarnock's Robbie McCrorie picked up a knock in the warm-up.
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Slicker conceded after just a minute on the pitch, with his kick picked up by Iceland, who scored through Andri Guðjohnsen, lobbing the ball over him.
He was then caught out from a corner, where Lewis Ferguson deflected the ball into his own net, before Victor Palsson scored with a header that bounced in off the goalkeeper's gloves.
The shot-stopper has since faced a lot of criticism on social media, with Clarke admitting that he feels sorry for him after the 3-1 defeat at Hampden Park.
Slicker earned his first senior international appearance before playing a league game for TownSlicker earned his first senior international appearance before playing a league game for Town (Image: PA)
"It was disappointing, the night didn't get off to a great start," the manager said after the game.
"We lost our goalie and compounded that by conceding the first goal.
"It's difficult for Cieran, I really feel for him. He got thrown into a situation that he wasn't quite ready for but Craig Gordon's injured, Liam Kelly's injured, Zander Clark is injured.
"It's something I thought might come back to bite us, the only good thing is it came in a friendly match."
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Scotland are in action again on Monday night, but they face a crisis in goal, with Clarke explaining that their "first five first-choice keepers" are all out. Does that mean Slicker will get his first start?
"Options not exactly jumping off the page at me," he explained. "We'll see how he [Slicker] is and make a decision.
"If you get injured in the first couple of minutes and then you come off I'd imagine [Angus Gunn] won't play the next game.
"I've had young Callan McKenna in the squad so he'll travel then we'll have a look and see if we can find another one in Scotland who's not on holiday."