26a is the only clue I have a question mark against.
If my reading of it is correct, the final letter of the answer added to the theme letter give a word "used to attract attention etc.", which I was initially satisfied with but, after looking up "invocation" in Chambers, it seems that, with the exception of the one definition related to law, all the others are to do with religion and used in prayer or when invoking a saint or deity, which couldn't be further from how, in my experience, I've heard this little word used. In fact, I'd go as far as to say that it's usually used as the precursor to some hostile act.
All this now makes me wonder if I've the wrong answer?