First, you should use fullfile to build paths rather than building them yourself.
filelist = dir(fullfile(Direc, [someprefix, '_*']));
Secondly, I would use numel instead of length since length behaves very differently with matrices. But in any case, x(1:length(x)) when x is the vector is the same as x, so the whole construct is pointless.
When you ask for a single field of a structure array, matlab returns a comma separated list. If you want to convert that into a single entity, you need to concatenate that list. As the elements of the list are strings, you need to concatenate them into a cell array, thus:
newfilelist = {filelist.name}
