\documentclass[12pt,twocolumn]{article}

\begin{document}
\begin{minipage}{\textwidth}
\onecolumn
\begin{abstract}
Some abstract. Full width.
Some abstract. Full width
Some abstract. Full width
Some abstract. Full width
Some abstract. Full width
Some abstract. Full width
Some abstract. Full width
Some abstract. Full width
\end{abstract}
\end{minipage}

\section{Stuff}
One columne here One columne here One columne here One columne here One columne here
One columne here One columne here One columne here One columne here One columne here
One columne here One columne here One columne here One columne here One columne here
One columne here One columne here One columne here One columne here One columne here
One columne here One columne here One columne here One columne here One columne here
One columne here One columne here One columne here One columne here One columne here
\end{document}