Want flush-left alignment and unnumbered equations, but 'flalign*' doesn't get the job done

The "fl" in flalign and flalign* stands for "full length", not "flush left".

In order to achieve your formatting objective, you should load the amsmath package with the option fleqn. (Here, "fl" really does mean "flush left".)

\documentclass{book}
\usepackage[fleqn]{amsmath}
\begin{document}
\begin{align*} % no equation numbering
\text{Where: } A &= \text{Apple}\\
               B &= \text{Banana}\\
               C &= \text{Cherry}
\end{align*}
\end{document}

You can define your own environment:

\documentclass{book}
\usepackage{amsmath}

\newenvironment{flushleftequation*}
 {\begin{equation*}\begin{lrbox}{\flusheqbox}$\displaystyle}
 {$\end{lrbox}\makebox[\displaywidth][l]{\usebox{\flusheqbox}}\end{equation*}\ignorespacesafterend}
\newsavebox{\flusheqbox}

\begin{document}

Some text before the conditions
\begin{flushleftequation*}
  \begin{aligned}
  \text{Where: } A &= \text{Apple}\\
  B &= \text{Banana}\\
  C &= \text{Cherry}
  \end{aligned}
\end{flushleftequation*}
Some text after the conditions.

\end{document}

Of course, you can do it with flalign, keeping in mind that it stands for “full length align” and not “flush left align”: just add something that will trigger full length.

\documentclass{book}
\usepackage{amsmath}

\begin{document}

Some text before the conditions
\begin{flalign*}
  \text{Where: } A &= \text{Apple} && \\
  B &= \text{Banana} \\
  C &= \text{Cherry}
\end{flalign*}
Some text after the conditions.

\end{document}

If you prefer a local left-flush (as I do), you may need to have a look at the package nccmath with its environment fleqn.

\documentclass{book}
\usepackage{mathtools}
\usepackage[showframe]{geometry} 
\usepackage{nccmath} % <-- load nccmath
\begin{document}
    \begin{fleqn} % <-- from nccmath package
        \begin{align*}
        \text{Where: } A &= \text{Apple}\\
        B &= \text{Banana}\\
        C &= \text{Cherry}
        \end{align*}
    \end{fleqn}
\end{document}