William, probably your best way to find the substitution is to google the first and last words from the extra letters along with one word from the title. That should bring up a Wiki article which will point you in the right direction.
As I've said before, I found the normal clues to be unsolvable in their own right and a certain amount of thematic matching to be needed. You've done better than me to solve one of them cold. What they have in common is not so much melodic as the first word from the extra letters (more on that in the Wiki article).
These clues are convoluted and I don't know how to hint at them without giving too much away. I mentioned earlier that the first (3-letter) word of 11/17 is defined as "a dear" in Chambers, and it's more commonly a noun that might describe what you feel on completing the puzzle. The horror film is the first word of 22/31 and is a very short title indeed. I hope that's of some use without giving too much away!