Taking to the stand at Circuit Criminal Court in Dublin, the bassist joked he was not used to being in front of a microphone.
His former PA, Carol Hawkins, is on trial for stealing cheques from the bassist over a four-year-period.
The 48-year-old, from Lower Rathmines Road in Dublin, worked for Clayton for 16 years from 1992.
The mother-of-two listened attentively as Clayton was grilled on their working relationship and details of his bank account.
She is accused of drawing 181 cheques from two of Clayton's accounts which she was a signatory for and lodging them into her own account.
Prosecution barrister, senior counsel Colm O'Briain, previously told the Circuit Criminal Court in Dublin that Ms Hawkins lodged the money into her own personal account, a joint account with her then husband John Hawkins and a credit card account, between 2004 and 2008.
The funds bought 22 horses, with more than 400,000 euro in cash listed as horse and horse expenditure, while thousands of euro were spent on exotic holidays and in designer boutiques in New York, such as Roberto Cavalli, the court heard.
Hawkins also bought a Volkswagen Golf for her son Joe, it was claimed, and paid for fashion and film courses which Clayton suggested may have been for her son and daughter.
"She had my absolute trust. We had been together a long time- working together. She had been very conscientious.
U2 bassist Adam Clayton
Clayton said Hawkins had carried out her duties "efficiently and well" when she worked for him.
"I trusted people, and that's why we're here today," he said.
"I felt she looked after my money and on many occasions accused others of being greedy so I was extremely surprised."
Clayton originally employed Hawkins as a housekeeper and her then husband as a driver and occasional chef. They were paid a joint salary of around 48,000 euro, a set-up Clayton described as "tax advantageous" for the couple.
The musician told a jury that Hawkins had confessed in 2008 to charging thousands of euro worth of flights to his account to visit her children in the US and London.
"She also mentioned that she had been suicidal and had taken an overdose," he went on.
"I was concerned for her health and recommended she see a therapist. I got her a therapist locally.
"In the matter of the money, I accepted she was a distressed woman. Her marriage broke up, her children had gone away.
"I said we would have to verify the amounts she had been claiming."
Clayton said he removed Hawkins as a signatory on his accounts, but kept her as an employee.
Defence barrister, senior counsel Ken Fogarty, put it to Clayton that the accused was authorised to spend money on more than just small household necessities with petty cash, that she was charged with making larger purchases for the mansion.
Disagreeing Clayton said: "I would buy the things I wanted around me, Carol Hawkins bought corn flakes."
He also denied claims from the defence lawyer that Hawkins oversaw the multimillion-euro refurbishment of his listed property.