Its hard to see what the purpose of this deletion is. It looks like you have multiple (499) datasets described by ixv (some kind of index) and by xc or xc_1 (the data). You want to delete values at the begining of the dataset where the index starts after the first index value of the last dataset (a=11).
I tried rewriting your code to be a little more followable. I used the find function to delete in one step instead of using a while loop.
I find that deleting values with a while loop can be problematic because the index of the while loop and the size of the array become out of sync as values are removed. I'm not sure if thats whats happening here though.
Tell me if this does what you're looking for. If not, let me know why, maybe i'll understand you better.
a = ixv{end}(1,1) % first value of last dataset
for k = 1:numel(ixv);
across = find(ixv{k}(:,1)<a,1) % find the first place where the index becomes larger than a.
across_1 = find(xc_1{k})<a-1,1) % same for xc_1 condition
% delete values before across
ixv{k}(1:across-1,:) = [];
xc{k}(1:across-1) = [];
xc_1{k}(1:across_1-1) = [];
end
