How does Clarisse define happiness in Fahrenheit 451?

Clarisse defines happiness as caring about things and living life on your own terms. 

For most of Montag’s society, happiness is defined as succumbing to mass entertainment and other mindless pleasures. This is why Beatty says that the firemen are bringing happiness to everyone, because they are destroying the books. Books make people think, and thinking makes them unhappy. Montag has never met anyone who thinks differently, until he meets his neighbor Clarisse.

Clarisse has a different view of the world. She doesn’t value speed and mass consumption of entertainment or drugs. Clarisse and her family are rebels. In an unusual conversation, she tells him that she is crazy along with all of her family, and that they stop and talk, take walks, and generally slow things down. This is quite a contrast to the way everyone else lives. Clarisse shocks him.

"Are you happy?" she said.

"Am I what?" he cried.

But she was gone—running in the moonlight. Her front door shut gently.

"Happy! Of all the nonsense."

He stopped laughing. (Part I)

It never occurred to Montag to wonder if he was happy or not. What is happy? What keeps people happy in his society is mindless pleasure. Does this make Montag happy? Thinking about it, he describes Clarisse as a mirror showing him himself. He wonders at her “incredible power of identification.” In only a few minutes, she made him question everything. 

Clarisse describes herself as crazy, but happy to be so. She doesn't fit in. She cares about people, and would rather talk, walk, sit, and look than take part in cruel or mindless activities. In later conversations Clarisse comments that Montag seems to actually be laughing.

"Did you look at the stretched-out billboards like I told you?"

"I think so. Yes." He had to laugh.

"Your laugh sounds much nicer than it did."

"Does it?"

"Much more relaxed." (Part I) 

Clarisse asked him if he was happy. She never actually says what happiness means to her, but her description of noticing the little things like the smell of the leaves and the sound of people talking shows that she is a stop-and-smell-the-roses kind of person. In a society where everyone goes fast, Clarisse gets her happiness from going slowly and thinking about things.