How do I align enumerate items inside empty description?

This must be a duplicate but I didn't see a good question to reference after a quick search so the easiest way without doing any complicated redefinitions of latex list code is

  \item [A Label:]\mbox{}\\[-1.5\baselineskip]

where the \mbox forces a paragraph start so the nested list indents normally.

You could also just do

\begin{enumerate}
\item[]
\item A1.
\item A2.
\item A3.
\end{enumerate} 

instead of the above. Works equally well (and looks better in my opinion).

The easiest solution I found for this was just \hfill at the end of the line.

\begin{description}
  \item[First] \hfill
    \begin{enumerate}
      \item A
      \item B
      \item C
    \end{enumerate}
  \item[Second] The second thing
  \item[Third] The third thing
\end{description}