CNN anchor Jake Tapper asked Sanders about that criticism during CNN's "State of the Union" on Sunday. Sanders wasn't having it.
"I haven't the vaguest idea what she's talking about," he said. "If she thinks that income and wealth inequality -- and the fact that the rich get richer while everybody else gets poorer -- is the only issue, it's not."
Even in this answer, you get a sense of what the criticism is about. Asked about criticism that his campaign is a single-issue campaign, he ... talks about that issue. His answer seems to betray an awareness of the criticism being thrown his way, while demonstrating that he often returns to the same talking points.
The question of whether Sanders is a "single-issue candidate" is ultimately one of semantics; Clinton and her supporters would argue that Sanders's economic proposals all broadly fall under the issue of wealth inequality, while Sanders seems to be arguing they are different issues.
But regardless of how you categorize his different arguments about the economy, there's no question that Sanders's candidacy is largely framed around a specific set of economic issues. In an almost Rubio-esque way, he has a habit of using the same phrases repeatedly in his speeches and debates: "A rigged economy," "all new income goes to the wealthiest 1 percent," and "people are working longer hours for lower wages."
His real challenge is to prove to voters that he has serious, developed ideas on foreign policy, in particular, and more broadly on issues beyond income and wealth, such as race.
In the end, whether or not Sanders is a single-issue candidate is up for debate, but his rhetoric has certainly left him open to such charges.