Many Spiritualists describe 2 archangels, St. Michael and St. Gabriel as ”二大天使”（ni dai tenshi) and also "三大天使" (san dai tenshi) means 3 archangels by adding St. Raphael to 2 archangels of the above. I have never heard "一大天使"(ichi dai tenshi) but I guess it means one of these archangels.

I agree w Skymama. Without any specific context....it's hard to tell. Maybe it's from some ANIME expression or something..?? We put 一in front of some words to show an example AND shorten the expression when possible, Like… “This is one of the products that we…blah blah blah…”, Goes like “これは……一製品(ichi seihin=product)です” which is same as “……一つの製品(hitotsu no seihin)です” “He was one of many artists who gathered at the cafe ” “彼はそのカフェにいた一画家(ichi gaka=artist)です” which is same as “……一人の画家(hitori no gaka)です” However, when “一大” itself is followed by some other nouns, this can mean something "huge/ of great impact/ great/ major/ mega”. Some phenomena or events that catch great public attention/ or a big event in a person’s life etc. “Summer Sonic は日本の夏の一大イベントです” =”Summer Sonic is a huge event that is held in Japan in ……”

The "ichi" MIGHT be chapter number or item number as you can see in this website: http://goseitai-to-seibo-no-sito.web.infoseek.co.jp/miosie%20no%20hon%2063.htm It's hard to decide what it really means without context, though.

As there are many archangels in different religion, I believe 一大天使 meant one of the archangels. It is hard to say without the real context though.