The space is there because inside the cell, a paragraph is first started, then it's ended and your itemize starts. You can wrap the code inside minipage to get the desired output:
\documentclass{scrartcl}
\begin{document}
\begin{tabular}{ll}
a & \begin{minipage}{5cm}
\begin{itemize}
\item sd
\end{itemize}\end{minipage}\\
\end{tabular}
\end{document}
However, I would prefer to make an environment of its own:
\documentclass{scrartcl}
\newenvironment{tabitemize}[1][5cm]{\minipage{#1}\begin{itemize}}{\end{itemize}\endminipage}
\begin{document}
\begin{tabular}{ll}
a & \begin{tabitemize}[5cm]
\item sd
\end{tabitemize}\\
\end{tabular}
\end{document}
Notice that I made the width as an optional arugument (I suppose you'll fix one width value). As well, I use the direct call to \minipage...\endminipage to avoid one unnecessary grouping.
According to suggest by azetina and A.Ellet I propose another version:
\documentclass{scrartcl}
\usepackage{tabularx}
\newenvironment{tabitemize}[1][\hsize]%
{\minipage[t]{#1}\begin{itemize}}%
{\end{itemize}\endminipage}
\usepackage{lipsum,showframe} % not needed
\begin{document}
\noindent
\begin{tabularx}{\linewidth}{@{}l@{}X@{}}
Hello & \begin{tabitemize}
\item \lipsum[1-2] % lipsum command just produces some dummy text
\item \lipsum[3]
\end{tabitemize}\\
\end{tabularx}
\end{document}
- We use
tabularx to use the whole width of a page.
- We set the argument to
minipage to \hsize which seems to be the right approach according to tabularx manual
- We set
[t] argument to minipage to get the right vertical alignment
- By
@{} we remove the (probably unnecessary) horizontal spaces.
