Now writing with a sheaf of paper propped up, covering that awful, repeating video...
Why do people only ask about the ones I've got?
LOL. It's either that, or everyone else thinks the ones I'm stuck on are pathetically easy, while I've got the ones they think are difficult.
I can understand what the theme is but it does seem terribly arbitrary. The added and removed letters are fine, but the "rhyming" words seem to be the answer some times, and omitted from the clue in others.
I'm still completely mystified by it. I know puzzles are easy if you know the answer :-), but I'm not sure if my difficulties are down to me or the compiler in this case.
I still think the first sentence of the instructions is gibberish. To what does 'not further defined' refer? A letter? Does that just mean there's a letter which isn't in the clue and is entirely unrelated to it? How does that work? Then I have to do an undefined something to it, based on something I've never heard of and can't find on the internet. This is just silly. And what is to be 'treated'? - a letter? An entire clue?
19A - The leading light is made up sequentially, but with a rhyming three-letter word for M added around the answer (or so I think).
I'm even more mystified by that. Taking the 2 letters I have (both A) and the 3-letter word for M in my Cockney list, I get... absolutely nowhere.
4D - remove a letter.
I assume it's removed from the last 4 (orig. 5) letters which are to do with 'smoke vent's opening', but I can't see what it is.
I can't parse 12A perfectly. I have the anagram (less 'y' presumably), but don't know how the first three and last two letters fit in. If it's a clue with a letter removed, then the rhyming doesn't come into it.
I was one step behind you, with the first 3 + last 2 left over but also OB - I assumed it wasn't a 'lose a letter' one. Still can't see how it works.
Sorry not to be any help.