Latex newcommand underlined section

I recommend you change the definition of \specsection to

\newcommand\specsection[1]{%
     \par\noindent{\raggedright
     \emph{\uline{#1}}
     \par}}

This requires loading the ulem package -- preferably with the option normalem.

---

A full MWE (minimum working example):

\documentclass{article}
\usepackage[normalem]{ulem}
\setlength{\parindent}{0pt} % just for this example
\newcommand\specsection[1]{%
     \par\noindent{\raggedright
     \emph{\uline{#1}}
     \par}}

\begin{document}
\sffamily % optional
Here you have normal text.
\specsection{Work package 1: One problem is that header lines sometimes become very long and you need two lines for them.}
Here you have normal text.
\end{document}