what's a replacement for \lefteqn in the align environment?

Use \MoveEqLeft from the mathtools package, like this:

\documentclass{article}
\usepackage{mathtools}
\begin{document}
\begin{align}
\MoveEqLeft \text{very very long} \\
 &= \text{shorter}
\end{align}
\end{document}

Edit: If you feel that this doesn't move the first line far enough to the left, then use, e.g., \MoveEqLeft[4] instead of \MoveEqLeft. This moves the first line 4em to the left; the default is 2em. See also page 17 of the mathtools documentation.

I prefer combining multline with aligned:

\documentclass{article}
\usepackage{mathtools}
\begin{document}
\begin{multline}
x=\\
\begin{aligned*}
 &= y\\
 &= z
\end{aligned*}
\end{multline}
\end{document}