So we have a garland, which sounds a bit (but not much) like an undefined word, from which we remove the necessary letter to obtain the entry. That is awful.
I cottoned onto the theme after about five minutes when I could see 1 across. I now have a complete grid fill, with one of the five unclued entries clearly relating to the theme, and the other four seeming to me to have nothing to do with it. However, the theme is surely the obvious one from 1ac and the ten redundant letters, and it is twelve letters. There seems to be no option for three of the remaining unclued lights but there is an ambiguity in the last.
Is this a matter of researching Wikipedia more, or am I missing something obvious?