Since Smack is a realistic portrayal of substance abuse and rebellion in youth, there is no clear-cut protagonist. Several characters tell the story from their own point of view. The two main characters, Gemma and Tar, are the major voices, and they act as parallels to each other. Tar is initially more sympathetic than Gemma, because he has been abused at home, but as the novel progresses, he becomes less sympathetic and more of an antagonist. Tar's addictive personality cause Gemma and others to suffer, and only at the end does he seem to understand that his actions have consequences.
Well, we argued and argued all over again. Tar wasn't interested. All he was willing to talk about was wheather we werre going to lend him the bus fare... he'd already given up in his mind.
"But you can do it, other people do it," andra kept saying.
"It's no worse than a dose of the flu," I reminded him.
"And I can't even cope with that," said Tar.
(Burgess, Smack, Google Books)
In this sense, since Tar is mostly wilfully-destructive while Gemma makes attempts to clean up and get off drugs, Gemma would be a better choice for protagonist. However, neither of them can be called a "hero" in any sense of the word; they are young and arrogant, and unable to see past the immediate moment. At best, they are both unsympathetic protagonists, while some of Tar's actions may veer into antagonistic territory.