A teenager accused of murdering Jodie Chesney told his co-accused he had "messed up" as he disposed of a knife and burnt his clothes, a court has heard.
Svenson Ong-a-Kwie, 19, denies stabbing Jodie to death in March.
Giving evidence at the Old Bailey, a fellow defendant, 17, said he went to Amy's Park, east London, with Mr Ong-a-Kwie, expecting him to do a drugs deal.
He knew the deal had "gone wrong" when he heard screams, jurors heard.
Before this, the boy said, he had not even been paying attention until he saw Mr Ong-a-Kwie running towards him.
'Something went wrong'
He told jurors he had spent the day with Mr Ong-a-Kwie before they were driven to Amy's Park by fellow defendant Manuel Petrovic.
When they arrived, the boy said, he stayed "a pace behind" as Mr Ong-a-Kwie approached "a group in the distance".
He assumed they were doing a drug deal, he said.
The defendant said he did not see what happened and after the screaming, he ran from the park with Mr Ong-a-Kwie.
"My first thought was a deal gone wrong, definitely something went wrong," he said.
After the group fled the scene, the boy said he asked Mr Ong-a-Kwie what had happened.
"He just said 'I messed up," he said.
"I asked what do you mean by that...you could tell he did not want to talk at that time."
He said they were driven by Mr Petrovic to Lawn's Park where Mr Ong-a-Kwie dropped a "silver, black panther sort" of knife on the ground.
"I knew straight away in the park the scream, us running off, obviously somebody has been stabbed," he said.
After an "awkward silence", jurors heard, Mr Ong-a-Kwie asked if he could stay overnight at the boy's house.
"I said obviously yeah let me ring my mum", the youth replied.
The defendant said Mr Petrovic came back with a bag of clothes for Mr Ong-a-Kwie to change into.
Mr Ong-a-Kwie then set light to the clothes he had been wearing, it was claimed.
The boy said he panicked when he saw news of the stabbing, and threw away the clothes he had been wearing on the night.
He told jurors he did not know the name Jodie Chesney and had never met any of her group of friends.
The defendant denied the claim that Ong-a-Kiwe had been preparing him to take over his drug dealing line so he could concentrate on becoming a professional boxer.
Mr Ong-a-Kwie, 19, Mr Petrovic, 20, the 17-year-old and another youth, aged 16, all deny murdering Jodie.
Jodie, 17, was with friends in Amy's Park in Harold Hill, east London on 1 March when she was stabbed in the back by one of two figures who then ran off.
The trial continues.