I was entering answer "In Extenso"in 9th row.
The bird loses L and becomes river. That's my entry at 17d.
The N clashes with second letter of 17d.
As you said, it will become clear later on.
'Tis...' is sort of &lit - the whole clue is the def (first 2 words are the anagram) I haven't found it in Chambers yet but it looks like 2 words possibly hyphenated.
I did not realize that the first clue answer can be ten letters long.
The preacher turns up 4-6.
I can enter first 4 and then have to solve other clues to decide what letters go in from Col. 4 to 9.
I will have to look up Messenet operas. Not familiar with them.
But is reversed in houses could be resides. Most grid lengths are 4 or 5. 8.9..
My answer to 4th clue Observes... is 8 letters long.