Custom glyph is not displayed properly

Why clipping? Just draw an arc!

\documentclass[a4paper, 12pt]{extreport}

\usepackage{mathtools}
\usepackage{amssymb}
\usepackage{tikz}

\usetikzlibrary{decorations.pathreplacing}
\usetikzlibrary{positioning}

\newcommand{\foo}[1][]{%
\begin{tikzpicture}[#1]%
\draw (0,0) arc(180:0:1.5ex);
\end{tikzpicture}%
}

\begin{document}
I love \verb|\foo| \foo
\end{document}

EDIT

I found out about the knots package. Have you tried it? It seems to produce exactly what you want. Type texdoc knots in the terminal for further info (or use CTAN)

BACK TO THE QUESTION

So, you have to extend the clipping area, in order to accomodate the line stroke.

I changed:

 \clip (-1.58ex,0.01) rectangle (1.6ex,1.6ex);

So:

\documentclass[a4paper, 12pt]{extreport}

\usepackage{mathtools}
\usepackage{amssymb}
\usepackage{tikz}

\usetikzlibrary{decorations.pathreplacing}
\usetikzlibrary{positioning}

\newcommand{\foo}[1]{%
\begin{tikzpicture}[#1]%
\begin{scope}
\clip (-1.58ex,0.01) rectangle (1.6ex,1.6ex);
\draw (0,0) circle(1.5ex);
\end{scope}
\end{tikzpicture}%
}

\begin{document}
I love \verb|\foo| \foo{}
\end{document}

Yields: