I think you might be able to appropriate the rules for grappling. Sure, WotC simplified the rules for 5e, but that doesn't mean you can't pull DM fiat; as others have stated, there aren't any rules specifically regarding this kind of situation to provide precedent.
If you can grab hold of your opponent and immobilize them, you or a companion might be able to tear off some armor or clothing; maybe not just any part, but a boot or a gauntlet or a hat/helmet could work. The necessary rolls could vary depending on what is being removed and how (fighters and rogues might try different methods, after all).
While the rules do state that armor takes about 10 minutes to fully equip/remove (others have already provided the references), that's for a full set of equipment; a single piece of armor or equipment such as a glove/boot/hat/helmet being removed in a haphazard way, if you aren't being careful not to harm the wearer or the item itself, might be removable from a pinned individual in a few rounds (probably more if they're struggling and roll well on their opposed checks).
For that matter, most hats aren't firmly attached to whoever is wearing them, so one of those could just fall off during the struggle if that's what you're after. (That may not apply to magic items depending on your setting, of course.)