Biden aides said he had been talking about ending fossil fuel subsidies, not the industry itself, when he spoke of a “transition” from oil, the BBC reported on Saturday.
The Democrat, meanwhile, again attacked Trump’s handling of coronavirus, saying: “He’s quit on America.”
Biden holds a steady lead in polls.
But the margin is narrower in a handful of states that could vote either way and ultimately decide the race on November 3.
More than 53 million Americans have already voted in person or by post in a record-breaking turnout driven largely by the coronavirus pandemic.
In the final moments of Thursday night’s primetime showdown in Nashville, Trump asked his challenger: “Would you close down the oil industry?”
“I would transition from the oil industry, yes,” responded Biden, adding, “because the oil industry pollutes significantly”.
Biden said fossil fuels had to be replaced by renewable energy over time with the US moving towards net zero emissions.
Democrats from the left of the party are enthused by the idea of the US ending its reliance on oil and gas as part of the fight against climate change.
But such a policy could turn off working-class voters in politically critical states such as Pennsylvania, Ohio and Texas, where many of the industry’s employees are based.