The quote
environment provides a group within which you can modify the \doublespacing
(from the setspace
package) to whatever you want. In this case, just add \onehalfspacing
in the quote
environment.
\documentclass{article}
\usepackage{setspace}% http://ctan.org/pkg/setspace
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\begin{document}
\doublespacing% <--- Global line spacing
\lipsum[1]
\begin{quote}
\onehalfspacing% <--- Local line spacing
\lipsum[2]
\end{quote}
\lipsum[3]
\end{document}
In addition to modifying the local line spacing within the quote
environment, you can include any font-related commands. For example, using
\onehalfspacing\small\itshape% Localized font modification
will typeset the quote
in small, italicized font: