documentclass{scrartcl}

usepackage[english]{babel}
usepackage[utf8]{inputenc}

usepackage[round]{natbib}
usepackage[hidelinks]{hyperref}

begin{document}

% this produces a warning, but no letters are cut off
subsection{Some text: citet{wilks} abcde}

% this does not, but the a is missing
subsection{Some text: texorpdfstring{citet{wilks}} abcde}

% same here
subsection{Some text: texorpdfstring{citet{wilks} } abcde}

begin{thebibliography}{99}
bibitem[Wilks, 2011]{wilks} Wilks DS. 2011.
   {em Statistical Methods in the Atmospheric Sciences/} (3rd ed.).
   Elsevier: Amsterdam.
end{thebibliography}

end{document}