Probably the posts from Mico's link are the best way to go, but here is an alternative that allows you to avoid typing the text twice: tikzmark. I actually do not think that this method is superior in this very case because

1. you have to load another package,
2. you need to insert a vphantom to get the vertical space right as an overlay tikzpicture does not insert space, and
3. you need to give each tikzmarknode a unique label.

However, it is an alternative, and as soon as you want a more fancy bracket (bent, dashed, whatever) this method is likely to continue to give you what you want.

documentclass{article}
usepackage{mathtools}
usepackage{tikz}
usetikzlibrary{tikzmark}
begin{document}
Some text to test the vertical space.

$
w = vphantom{overbracket[0.75pt]{a}^{x}}% to fix the vertical space
    underbracket[0.75pt]{aatikzmarknode{a1}{a}dots a}_{n}
    underbracket[0.75pt]{atikzmarknode{a2}{a}adots a}_{m}
    underbracket[0.75pt]{bbbdots b}_{n}
    underbracket[0.75pt]{aaadots a}_{m}
    underbracket[0.75pt]{bbbdots b}_{n}
    begin{tikzpicture}[overlay,remember picture]
     draw[line width=0.75pt] (a1.north east) -- ++ (0,0.8ex)
      -| node[pos=0.25,above]{$x$} (a2.north east);
    end{tikzpicture}
$

Some text to test the vertical space.
[w = vphantom{overbracket[0.75pt]{a}^{x}}% to fix the vertical space
    underbracket[0.75pt]{aatikzmarknode{a3}{a}dots a}_{n}
    underbracket[0.75pt]{atikzmarknode{a4}{a}adots a}_{m}
    underbracket[0.75pt]{bbbdots b}_{n}
    underbracket[0.75pt]{aaadots a}_{m}
    underbracket[0.75pt]{bbbdots b}_{n}
    begin{tikzpicture}[overlay,remember picture]
     draw[line width=0.75pt] (a3.north east) -- ++ (0,0.8ex)
      -| node[pos=0.25,above]{$x$} (a4.north east);
    end{tikzpicture}
]
Some text to test the vertical space.
end{document}