One of Tom ap Rhys Pryce's killers claimed his partner-in-crime had him beaten up in jail.
Carty (left) was accused of getting Brown beaten up
Delano Brown, 18, said three other youths attacked him at Feltham young offenders institute during the Old Bailey trial.
Donnel Carty, 19, was accused of laughing while Brown was punched and kicked after a CCTV camera had been concealed with a jumper.
Carty had been giving evidence earlier in the day and had been kept with the other youths in the court during the lunch recess.
He was later recalled by a judge and asked about the incident, but denied instigating it.
Mr Justice Aikens chose not to reveal the incident to the jury, as he feared it may have prejudiced the case.
Brown said one of his attackers said: "You are snitching on your co-d (co-defendant)."
Brown said he returned to Feltham with Carty and the three accused of attacking him. They were given an evening meal and locked up in a room.
He told the jury: "As soon as the door was locked by the prison officer, the fourth man covered the CCTV camera with a jumper.
"I was trying to eat my food and I felt someone behind me and somebody hitting me from behind. I was trying to get up and I saw a couple of people running towards me. I
tried to defend myself. I ducked and covered my head.
"The inside of my right ear was pouring with blood. I tried to get out of the
door. I was hit in the face again."
Brown was rescued by a prison officer who unlocked the door. The incident is being investigated by Feltham authorities.
Carty was questioned by Brown's defence lawyer Courtenay Griffiths QC, who asked: "You organised that, didn't you?"
Carty replied: "No." He blamed the attack on an argument Brown had with the youths earlier.