user34241 (imported from SE)
%I

\documentclass[ aspectratio = 1610 ]{beamer}

\usepackage{amsthm}

\usepackage{amsmath}

\usetheme{Frankfurt}

\makeatletter

\setbeamertemplate{theorem begin}{%

\begin{\inserttheoremblockenv}%

}
\setbeamertemplate{theorem end}

{\end{\inserttheoremblockenv}}

\makeatother

\begin{document}

\begin{frame}

\begin{definition}[ Is it possible to produce this heading without the white bar below?]

\end{definition}

\end{frame}

\end{document}
samcarter
Removing an empty body from a rounded beamer block is not trivial, however you can simply reverse roles and use a block with empty title:


\documentclass[ aspectratio = 1610 ]{beamer}

\usetheme{Frankfurt}

\makeatletter

\setbeamertemplate{theorem begin}{%
\begin{\inserttheoremblockenv}%
}
\setbeamertemplate{theorem end}
{\end{\inserttheoremblockenv}}

\makeatother

\begin{document}

\begin{frame}

\begin{definition}[ Is it possible to produce this heading without the white bar below?]
\end{definition}

{
\setbeamercolor{block body}{bg=structure.fg!75!black,fg=white}
\begin{block}{}
\inserttheoremheadfont Is it possible to produce this heading without the white bar below?
\end{block}
}

\end{frame}

\end{document}


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