Clark's first experience of professional acting came in 2008 when she starred alongside Dakota Blue Richards in the BBC film Dustbin Baby, an adaptation of the Jacqueline Wilson novel of the same name. She played the part of Poppy, a teenager with Asperger syndrome. The BBC said that the fact Clark also had the condition offered her a "unique take" on the role. Clark was the first actress with Asperger syndrome to portray a fictional character with the condition. Clark auditioned for the role when her mother, Nicky, saw the position advertised on a website about autism. She said that she thinks "it's incredibly positive that the BBC chose to find an actress who has the same condition as the character." Clark said that being on set was the best experience of her life, and though "at first it was a bit intimidating", she said that eventually you "stop noticing the cameras". She said that her "Asperger's made some things on the film set difficult at first, like dealing with the sudden noise of the storyboard, but I was soon so focused on acting that I didn't notice anything else."
Prompted by Clark's role in Dustbin Baby, Clark's mother started the Don't Play Me, Pay Me campaign in attempt to stop actors "playing disabled". Her mother said that actors without mental disabilities playing characters with specific conditions is the "blacking-up of the 21st century", claiming that "we need tobreak down these barriers. They're unacceptable and indefensible in a modern-day society, especially when there are so many good, disabled actors who are both ready, eager and able to take on these parts". Clark is heavily involved in the campaign, and said that
It is not just mentally disabled actors who lose out when non-disabled people are employed to act them. Audiences think they are getting an authentic portrayal of a mentally disabled person, but they're not. It's not like putting on a different accent or learning what it was like to be raised in a different era. You can't understand what it is like to have a mental disability unless you've really lived with it. When non-disabled people try to portray us, they tend to fall back on stereotypes that have done our community so much harm in the past.
Targets of the campaign include setting up a forum for mentally disabled actors, and "to see disabled actors playing parts where the least interesting thing about them is their disability." As part of the campaign, her mother, who was an aspiring actress herself, asked stage schools to be more pro-active in encouraging the enrollment of students with disabilities.
