Speaking to Now as he stripped off for the racy images, Clark confessed that he used to take boxing lessons twice a week when he was 12 and "can fight if someone starts" on him.
"At school, a boy bullied me for three years," he recalled. "One day, I knocked him flat out. No-one said anything to me again. But fighting isn't the answer."
Elaborating on the declaration of love made by Williams, who has been a vocal supporter of his, Clark explained: "Robbie said to me, 'You don't realise - I want you in my life one way or the other'.
"I was like, 'Godparent?' And he said, 'Sorted'. I said, 'If it doesn't work out with [Robbie's wife] Ayda [Field], you and me will run away together'. And he said, 'I'm bang up for that'. So watch this space!"
Clark also said that his apparent feud with Williams's Take That bandmate and X Factor judge Gary Barlow is only for show.
"On stage he's Borelow, but off stage we get on really well," the Essex singer said. "I call him Banter Barlow. I love Gary and have so much respect for him... I think he's got a soft spot for me.
"He's a genuine bloke. If I get the chance to release a single, he's got to be involved. I might get him to write it - or star in the video."
Gary Barlow and Christopher Maloney during this week's X Factor results show
However, Clark seemed to have few kind words to say about fellow contestant Christopher Maloney, claiming that stories of his diva behaviour backstage "have substance to them", and vowing that "the truth will out".
"It's a really stressful environment, but I don't like seeing people spoken to like s**t," he said.
"And if I do, I say something about it. I'm very proud that I'm the same boy I was when I started. I don't need a sob story or to be rude to get what I want. The truth will out."
Read the full interview in Now magazine, out now.