\begin{frame}{Frame 1}
\begin{itemize}
\item A \pause
\item B \pause
\item C \pause
\showsubframe
% continue here
\item D \pause
\item E \pause
\end{itemize}
\end{frame}
\begin{frame}{Subframe} % only show this during Frame 1, not also after
% something to help explain point C from Frame 1
\end{frame}
\frame<1-3>[frame1]{
\begin{itemize}
\item<1-> A
\item<2-> B
\item<3-> C
\item<4-> D
\item<5-> E
\end{itemize}
}
\frame{
% something to help explain point C from Frame 1
}
\againframe<4->{frame1}