Co-trainer Tom Charlton has claimed his first Group 1 with Linebacker defeating Broadsiding in the Randwick Guineas.

Linebacker has delivered on his potential in the Randwick Guineas, upstaging star colt Broadsiding to give his co-trainer Tom Charlton his first Group 1 victory.

The James Cummings-trained Broadsiding was the $1.60 favourite to land his fourth Group 1 win in Saturday's 1600m feature for three-year-olds but jockey Zac Lloyd was able to execute a pre-race plan and had Linebacker ($9) in a position to make Broadsiding and Swiftfalcon ($5) have to chase him in the straight.

Tenbury Wells rolled along in front with Lloyd tracking him on Linebacker before taking over early in the home straight.

Broadsiding emerged as a key challenger but Linebacker kept finding to deny the favourite by a half-length, with Swiftfalcon another neck away third.

Charlton, who hails from England, joined the John O'Shea stable in 2019 before being elevated to co-trainer in the middle of last year, and he said it was a surreal feeling to win his first Group 1 as a trainer.

"I don't know what to say. It's just unbelievable," Charlton said.

"I'm just so thankful to my boss and all the clients. It just feels surreal, it really does.

"We were frustrated in the spring. We thought he could be a Group 1-winning colt and that didn't go to plan and we were convinced he was better than what he was showing in his first two runs back.

"He got into a rhythm today and got a bunny to follow and he was strong. You could see turning in he was going to give it a shake.

"He's always been a very talented horse."

For O'Shea, it was his 30th win at Group 1 level and his first since Lion's Roar took out the same race four years ago.

He said it was the plan to have Linebacker third-up in the Randwick Guineas, with the gelding finishing runner-up in the Eskimo Prince Stakes first-up before a sixth to Broadsiding in the Hobartville.

"We were quietly optimistic," O'Shea said.

"You never want to be too optimistic against a great horse like Broadsiding but I said to the boys earlier in the season, I thought last year when Militarize got beaten he was second-up and I think if he was third-up he wins.

"I wanted to go into it third-up and see if we could out-tough them. That was our plan today and it probably worked in our favour.

"It's exciting for Tom. It's great because he's a great young lad and he's brought so much to our organisation.

"He actually stayed to buy this horse as a yearling. We all went home and he stayed at Karaka in the cold waiting to buy him, so it's very special for him."

Lloyd had ridden Broadsiding to victory in the Hobartville two weeks ago but was on Linebacker in the Guineas, with champion jockey James Mcdonald taking over on Broadsiding.

Lloyd said the race panned out perfectly for him.

"My pre-race strategy was to get both Broadsiding and Swiftfalcon uncomfortable," Lloyd said.

"I feel like Linebacker is a superior miler and if we left it to a sit-and-sprint they're probably sharper than him.

"Broadsiding got to me at about the 200 (metre mark) and I thought, 'that's a bit sooner than I would have liked'. He got to about a neck and my bloke held him quite comfortably the last 100 metres so it was a very good effort."

McDonald said Broadsiding went "really well".

"He just found it a bit tough to peg back the winner," McDonald said.