\documentclass{article}
\def\eatone#1{}
\begin{document}
\section{Testing}
\expandafter\def\expandafter\baz\eatone##1{(#1)}

Expecting (beep) : \baz{beep}

\expandafter\def\expandafter\bloop\expandafter{\baz{foo}}

Expecting (foo) : \bloop

\def\bar{(Bar)}
Expecting (Bar): \bar

{ \def\bar{BAR}
Expecting (BAR): \bar }

Expecting (Bar): \bar

{ \global\def\bar{BAR}
Expecting (BAR): \bar }

Expecting (BAR): \bar

\end{document}
