Align over many rows in table

The following solution is a refinement of @JohnKormylo's suggestion: change the tabular setup from 2 to 3 columns, and have LaTeX insert = (type mathrel, in math mode) automatically between columns 2 and 3. That way, the spacing around the = symbol will automatically be appropriate for math-mode material.

\documentclass{article}
\usepackage{array,amsmath,multirow}
\newcommand\mc[1]{\multicolumn{2}{c|}{#1}} % handy shortcut macro
\begin{document}

\begin{table}[htbp]
\centering
\begin{tabular}{| c | r @{${}={}$} l |}
\hline
\multicolumn{1}{|l|}{a} & \mc{looooooooooong} \\ \hline
\multirow{3}{*}{cc} & a       & 1    \\ \cline{2-3}            
                       & abbbbbb & 5234 \\ \cline{2-3}            
                       & csf     & 6    \\ \hline
\end{tabular}
\end{table}
\end{document}

How about a solution with only two rows, and an aligned environment? For the horizontal rules inside the aligned environment, I use the capability of \cmidrule to extend on both sides with the (lr) argument. This is obtained with a rather simple code. The 0.95em lengthening of \cmidrule was found by trial and error:

\documentclass{article}
\usepackage{array,amsmath}
\usepackage{booktabs}

\begin{document}

\begin{table}[htbp]
\centering\setlength{\aboverulesep}{0pt}\setlength{\belowrulesep}{0pt}
\begin{tabular}{| c |c|}
\hline
a & looooooooooong\\ \hline
c & $\begin{aligned} a & = 1 \\
\cmidrule(l{-0.95em} r{-0.95em}){1-3}
                       abbbbbb & = 5234 \\
\cmidrule(l{-0.95em} r{-0.95em}){1-3}
                       csf & = 6 \end{aligned}$ \\ \hline
\end{tabular}
\end{table}

\end{document} 

Here are two ways using overlapping of content (since it looks like it should be math, I've used \mathllap and \mathrlap from mathtools), or boxing them into similar-sized boxes (using eqparbox):

\documentclass{article}

\usepackage{mathtools,eqparbox}

\begin{document}

\begin{tabular}{|c|c|}
  \hline
  a & looooooooooong   \\
  \hline
    & $a = 1$          \\
  \cline{2-2}
    & $abbbbbb = 5234$ \\
  \cline{2-2}
  c & $csf = 6$        \\
  \hline
\end{tabular} \qquad
\begin{tabular}{|c|c|}
  \hline
  a & looooooooooong   \\
  \hline
    & $\phantom{abbbbbb}\mathllap{a} = \mathrlap{1}\phantom{5234}$   \\
  \cline{2-2}
    & $abbbbbb = 5234$ \\
  \cline{2-2}
  c & $\phantom{abbbbbb}\mathllap{csf} = \mathrlap{6}\phantom{5234}$ \\
  \hline
\end{tabular} \qquad
\begin{tabular}{|c|c|}
  \hline
  a & looooooooooong   \\
  \hline
    & $\eqmakebox[lhs][r]{$a$} = \eqmakebox[rhs][l]{$1$}$    \\
  \cline{2-2}
    & $\eqmakebox[lhs]{$abbbbbb$} = \eqmakebox[rhs]{$5234$}$ \\
  \cline{2-2}
  c & $\eqmakebox[lhs][r]{$csf$} = \eqmakebox[rhs][l]{$6$}$  \\
  \hline
\end{tabular}

\end{document}