This was a bit toughish, with four or five answers escaping me until the third sitting just now.
The fact that no clue pushed my "That's not square-dealing à la Macnutt!" buttons is always a good sign.
I had never heard the answer to "Fight vulgarisms in grand source of illumination"; in fact I never before encountered that first word, but since the correct answer was the only possibility (cue Sherlock Holmes quote), it had to be right.
Several clues I have never parsed (e.g., 25A) but expect to find them explained when I browse previous posts.
16D was Really. Devilish.