[Tex/LaTex] Enumerate margin and linespread

Question

How do I go about defining an enumerate environment (using the enumitem package) (or possibly another environment?) such that the distance from the left side of the text to the LABEL is, say, 5 cm. Using

\begin{enumerate}[leftmargin=5cm]

makes 5 cm to the text inside enumerate, not to the label "ARG."

Also, how do I define the linespread inside enumerate to be 1, such that I still have a blank (1.5) line above the environment?
This code:

\begin{enumerate}
\linespread{1}\selectfont

messes up the linespread above.

I'm using the memoir class, the command \linespread{value} to change spacing, and I'm using the enumitem package.

The picture below should display what I want.

Here is an example that shows what's wrong:

\documentclass{memoir}
\usepackage{enumitem}
\linespread{1.5} 

\newenvironment{test}[1]{%
  \begin{enumerate}[leftmargin=2cm,rightmargin=2cm]%
    \linespread{1}\selectfont%
    \item[#1.]}{%
  \end{enumerate}
}

\begin{document}
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Proin lacinia tristique arcu sed posuere. Mauris semper dui lobortis, commodo quam a, interdum est. Donec lobortis dapibus massa, ut cursus velit. Donec egestas eget sem ut vehicula. Morbi in est vehicula libero hendrerit luctus eu egestas ante.
\begin{test}{ARG}
Cras vestibulum tincidunt pretium. Fusce gravida leo lacus, nec condimentum mi iaculis quis. Mauris maximus lorem felis, in fringilla magna commodo laoreet.
\end{test}
Vestibulum sodales consectetur porttitor. Ut tincidunt metus a nunc maximus, sed iaculis arcu commodo. Aliquam nibh massa, pretium vitae orci vel, lacinia sodales libero. Nunc quis ex id purus bibendum aliquet pulvinar et sem. Sed a rutrum tortor. Nullam tempor leo eu tristique auctor.
\end{document}

which gives (notice the wrong spacing between the text and the enumerate environment):

Answer 1

I assume you are using \usepackage[onehalfspacing]{setspace} to get your document set with one-half spacing. A good package to modify lists is enumitem. This might solve your problem:

\documentclass{article}
\usepackage[onehalfspacing]{setspace}
\usepackage{enumitem}
\usepackage{blindtext}
\begin{document}
\blindtext
\begin{singlespace}
  \begin{enumerate}[leftmargin=5cm]
    \item \blindtext
    \item \blindtext
\end{enumerate}
  \end{singlespace}
\blindtext
\end{document}