Stewart could make his international debut on Thursday night when Poland visit Hampden Park in a friendly, marking a remarkable rise for the player.
The 25-year-old leads the League One goalscoring charts this season with 22 goals, and was called up to the squad when QPR and Hibernian strikers Lyndon Dykes and Kevin Nisbet were ruled out through injury.
Clarke was asked at his pre-match press conference what had caught his eye from Stewart, and the Scotland boss noted the impressive goalscoring feats in a pressurised situation.
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“I could say he’s an Ayrshireman and it’s always good to get another in the squad,” Clarke joked.
“But he’s come up the hard way, released as a kid from St Mirren and then playing in the Ayrshire juniors.
“He had quite a quick rise when he did well at Ross County, scored a few goals and got his move to Sunderland.
“I like the way he moves on the pitch and I like the way he scores goals.
“People might say, ‘He’s only scoring at League One level’, but you have to realise he is playing for a big club with big demands,” Clarke added.
“Sunderland are definitely in the wrong league and there is always pressure at a club like that. But he’s produced the goods with the goals.
“I knew I wasn’t going to have Lyndon Dykes for this game and Kevin Nisbet has picked up a serious injury, so it was just the chance to look at another forward.
“The most important part is for Ross to go away from this camp and think, ‘I liked that. I want to do it again’.”
Stewart told safc.com that receiving the call from Clarke was the highlight of his career so far, and that he is embracing the challenging of training and competing with a number of Premier League players.
“I received the call on Saturday night when we were on the team bus heading back from Lincoln,” he said.
“Alex Neil said to me that he had spoken to Steve Clarke and that Scotland were going to call me up, explaining that he had passed on my details and that I should receive a phone call shortly.
“About 15 minutes later, Steve called to tell me officially and it was probably the highlight of my career so far, as I don’t think there is anything better in football than representing your country.
“The step up will allow me to get a gauge for that level and to be able to train with the other players in the Scotland squad – many of whom play in the Premier League – is going to be massive for me, as will learning from a top manager in Steve Clarke.
“Overall, it will be a great experience and one I’m really looking forward to.”
Scotland will play another friendly next week before Stewart returns to Sunderland action.