I admire that you're using non-standard indicators; the risk, of course (which I've run into myself) is that people won't parse them properly. Like, 1D is a fun homophonic twist on XL (could also have gone with "forty") and I admire that 1A references the same mechanism for MT, even if spelling out "one down" is a particularly devious and uncommon way to cross-reference a clue. 2D is great (even if "joke language" isn't exactly how I'd describe Pig Latin) and I'm wondering if 3D actually requires some devilry. Even 5A, your easiest clue, uses the less common "prime" indicator.
And then finally, 4A. I think you need to be really careful with clues like this. Not everybody knows the game Undertale, and of those who do (like me), even fewer are going to recognize "They fell into the monster's realm" as a reference. If you must go this route, I would advise making the cryptic part entirely clear: I'm not sure that "Burn marks" equals CHAR (I think that'd be something more like "Leave burn marks") and I'm not sure how to get "A" from "centerpiece" (I want that to be an E, as in the center of "piece"). Apologies if I'm misparsing something much more obvious.
Thanks for the feedback! I agree that 4A is too difficult; I was too attached to my down clues and that was the best across possibility to me. The intended parsing for that one is "Burn marks on" being CHAR and "the centerpiece?" being the letter both at the center of the grid and the center of the entry. I should have placed an indicator on the center square to make it slightly more fair. I'm also starting to think that the intended read for the completion of 3D, "Music theory has us play b[y ear n]ow," probably isn't fair in itself even if the solver knows what printer's devilry puzzles are. I think I should have tried to change 3D to get a better 4A, perhaps YEMEN and CLAIM or CHARM, and come up with more intuitive clues for them.
That said, the only way to learn what works and what doesn't—both for yourself and for your audience—is to try them out. I think the devilry clue was fairly indicated, I just suck at that particular form of puzzle. (There are those who can't stand Spoonerisms; I still love 'em.) I look forward to see what comes next. (And in retrospect, "burn marks on" is fair for CHAR. I just thought the "on" was doing something else.)