It's not "leftist" necessarily, but all leftists should inform themselves about the Russian Revolution IMO, which is covered by season 10 (the final and currently ongoing season) of the Revolutions|Spotify podcast.
Mike Duncan isn't explicitly leftist or anything, but he really does his homework and portrays things in a really neutral way. Whether or not you are a big stan of the USSR and what came after, the Russian Revolution was the most successful attempt at overthrowing capitalism (to an extent), and any future movement should learn from and analyze all aspects of what happened in those years.
Citations Needed|Spotify is my other favorite with their in-depth media criticism (if you like Chomsky's stuff, you'll probably like what they have to say).
"Well there's your problem", about engineering disasters - it's also a YouTube channel but there are podcast feeds and it works as audio (that's how I listen). Also "Know your enemy" is a leftist look at the right.
Reasons to be Cheerful isn't necessarily a 'leftist' podcast (One of the hosts is the former leader of the Labour party in Ed Milliband), but the show spends a lot of time talking about leftist ideas to improve the world.