The "Vanderpump Rules" star took to social media Thursday to clear up the legal action he took against AM -- where he countersued her in the case Rachel Leviss brought against him and Ariana regarding alleged revenge porn -- and made it clear ... this was a mistake.

Tom claims one of his attorneys told him about a cross-complaint -- and, allegedly, told him what he was about to file is customary ... with Tom saying he urged him to sign off on it, which Tom says he did ... albeit, unwittingly, or so he insists.

He adds, "The words 'New Lawsuit' or 'Suing' were not articulated to me. I should've done more of my due diligence on the matter." TS goes on to say that upon realizing what had been done, he booted the attorney in question.

Tom adds, "In no way am I suing Ariana. The action against Ariana brought on my behalf is being removed. I had no ill will or vindictiveness toward Ariana."

He finishes by saying now that he's reversing the filing ... he's hoping to get through this case much more quickly, noting he and Ariana simply want to move on past the "Scandoval." Indeed, it's been a long ride ... and his apparent salvo Wednesday came out of nowhere.

As we reported ... Tom countersued Ariana -- within Rachel's civil case -- and he leveled serious allegations against Ariana ... including the notion that she allegedly made copies of Rachel's nudes from Tom's phone and disseminated them without permission.