I wrote an array environment inside align. As array have a blank space around it, I cant align it properly.
I'm using array{l}, because I do want elements on the left inside array.
\documentclass[]{report}
\usepackage{amsmath}
\begin{document}
\begin{align*}
a=&a
\\
a=&\begin{array}{l}
bb
\\
ccc
\end{array}
\end{align*}
\end{document}
Now I think to understand what you want: the matrix* environment from the mathtools package.
\documentclass[]{report}
\usepackage{amsmath}
\usepackage{mathtools}
\begin{document}
\begin{align*}
a&=aaaa
\\
a&=\begin{matrix*}[l]
bbb
\\
cc
\end{matrix*}
\end{align*}
\end{document}
&=
Remove the padding.
\documentclass[]{report}
\usepackage{amsmath}
\begin{document}
\begin{align*}
a&=a
\\
a&=\begin{array}{@{}l@{}}
bb
\\
ccc
\end{array}
\end{align*}
\end{document}
For correct spacing around the = sign, the & should go in front of it.
How to align entries of a matrix? That way it may get more attention, and I do not think that the problem is related to the math environmentalign. But this is all up to you.\begin{array}{@{}l@{}}.