\begin{tikzpicture}[>=latex, node distance=1cm, on grid, auto]
  \tikzstyle{node}=[fill=white, text centered]
%  \begin{scope}
    \node [node] (f) {$f$};
    \path (f)+(-0.6,-1)  node (x) [node] {$x$};
    \path (f)+(0.6,-1)  node (g) [node] {$g$};
    \path (g)+(0.0,-1)  node (a) [node] {$a$};
    %\path (g)+(0.7,-1)  node (z) [node] {$z$};
%  \end{scope}
  % Draw arrows between elements
	  \path [draw, -] (f.south)+(-0.2,0.0) -- node	[above] {} (x);
	  \path [draw, -] (f.south)+(0.2,0.0) -- node	[above] {} (g);
	  \path [draw, -] (g.south)+(0.0,0.0)-- node 	[above] {} (a);
	  %\path [draw, -] (g.south)+(0.2,0.0)-- node		[above] {} (z);
\end{tikzpicture}
%\caption{}
%
%f(x,g(y,z))
%
%  f
% / \
%x   g
%   / \
%  y   z