You don't mention it explicitly, but I assume you're using the enumitem package.
The following code should work for you; if you want the cross-reference to a second-level item to also include the "number" (here: letter!) of the higher-level item, you can do so by specifying the desired format explicitly via a ref= ... statement.

\documentclass{article}
\usepackage{enumitem,hyperref}
\hypersetup{colorlinks=true}
\setlength{\parindent}{0pt} % just for this example
\begin{document}
\begin{enumerate}[label=\normalfont({\alph*})]
\item abc\label{itm:a}
\item def\label{itm:b}
\begin{enumerate}[label=\normalfont({\roman*}),ref=\alph{enumi}({\roman*})]
\item uno\label{itm:i}
\item dos\label{itm:ii}
\item tres\label{itm:iii}
\end{enumerate}
\end{enumerate}
Here's a cross-reference to item \ref{itm:i}.
Here's another cross-reference, to \autoref{itm:iii}.
\end{document}