After returning from a broken leg last season, the defender has only featured four times this season, last coming off the bench on January 18.
The 22-year-old has started once this campaign, in the 2-1 loss to Mansfield on October 26, and Clarke is clear on what he wants from the centre-back.
“Do a lot better in the reserves,” he said.
“I like Kory as a lad, I’ve had a good conversation with him and he knows where I stand on things.
"Kory is a good lad but he needs to raise his standards that I set and he’s working hard to do that.
"You’ve got to prove it to me and to my staff on a consistent basis and Kory’s got to do that to try and earn himself a new deal in the summer.”
Clarke has been keen to give academy products a chance to shine this season, with Alfie Bates, Sam Perry and Joe Willis all having opportunities.
Mitch Candlin, who had a spell on-loan at Nuneaton earlier in the season, and 15-year-old striker Luke Pearce are now vying for their own chance and Clarke has hinted that there is scope for them to be involved.
“Mitch knows where I stand with him, we’re monitoring him and having a look at his reserve performances,” he said.
“At times he’s done well, he’s covered the distance, he’s working hard.
“I can’t fault the young lad’s work ethic and desire.
“Luke Pearce is a 15-year-old kid who we think is a really good player – he’s looking bright and sharp.
“He’s trained with the first team and you never know he might get a run out before the end of the season, we’ll see how things progress.
“We’ve had a lot of positives with Sam Perry and Joe Willis who have trained with the first team on a regular basis.
“We’re really pleased that one or two of the young lads are stepping up to the plate.”
Left-back Callum Cockerill-Mollett was another name to emerge this campaign, but after suffering a hamstring injury away at Salford in January, Clarke confirmed he still has a way to go with his recovery.
“He had a bad injury and he’s still a good few weeks away to be honest,” he added.
“Which is unfortunate because Callum was hitting a bit of form when he came into the team.”