Accent in letters over a Letter

\documentclass{article}
\usepackage{stackengine,graphicx}
\begin{document}
$\ensurestackMath{\stackengine{.5pt}{\tau}{\scriptscriptstyle e-m}
  {O}{c}{F}{F}{S}}$
or
$\ensurestackMath{\stackengine{1pt}{\tau}{\scalebox{.5}{$\scriptscriptstyle e-m$}}
  {O}{c}{F}{F}{S}}$
\end{document}

With a definition like \newcommand\overtext[2]{\ensurestackMath{\stackengine{1pt}{#1}% {\scalebox{.5}{$\scriptscriptstyle #2$}}{O}{c}{F}{F}{S}}}, a simple invocation of $\overtext{\tau}{e-m}$ accomplishes the task.

If the overset is actually text over a long tilde...

\documentclass{article}
\usepackage{stackengine,graphicx,scalerel,wasysym}
\newcommand\overtext[2]{%
  \savestack\tmp{$\scriptscriptstyle \reallywidealttilde{#2}$}
  \ensurestackMath{\stackengine{.5pt}{#1}%
    {\scalebox{.5}{\tmp}}{O}{c}{F}{F}{S}}}
\newcommand\reallywidealttilde[1]{\ThisStyle{%
  \setbox0=\hbox{$\SavedStyle#1$}%
  \stackengine{.5pt}{$\SavedStyle#1$}{%
    \stretchto{\scaleto{\SavedStyle\mkern.2mu\AC}{.5150\wd0}}{.6\ht0}%
  }{U}{c}{F}{T}{S}}}
\begin{document}
$\overtext{L}{e-m}$ versus $\overtext{L}{x}$ versus $\overtext{L}{Ax^2 + Bx + C}$
\end{document}

The \reallywidealttilde macro is based on my answer at Big tilde in math mode.

Here are three additional possibilities. They vary in (a) the symbol used to connect "e" and "m" -- a mathematical minus sign or a simple dash -- and (b) the size of the superscript material -- \scriptsize or \tiny. Not knowing what "e-m" stands for, I don't dare offer an opinion on which form is best.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
$\overset{\mathit{e-m}}{\mathcal{T}}$
$\overset{\textit{e-m}}{\mathcal{T}}$
$\overset{\textit{\tiny e-m}}{\mathcal{T}}$
\end{document}

I see it as L with a tilde, but it can easily be changed to tau.

\documentclass{article}
\usepackage{stackengine}
\newcommand\tauem{\widetilde{\rule{4pt}{0pt}L\rule{4pt}{0pt}}\raisebox{1.1em}{\kern-15pt\tiny\ensuremath{e-m}}}
\begin{document}
$A\tauem B$
\end{document}