Decrease spacing between lines of paragraph in longtable.

You can also set line spacing locally in a clean way as this:

\documentclass{article}
\usepackage{longtable}
\usepackage{setspace}
\begin{document}

\begin{spacing}{.7}
\begin{longtable}{ l | l | p{5 cm} }
    a & b & a small phrase \\
    a & b & a small phrase \\
    a & b & here is a long sentence which wraps to the next line, here is a long sentence which wraps to the next line \\
    a & b & a small phrase
\end{longtable}
\end{spacing}

\end{document}