Hi,

sorry, I do not get the question. Why do you say that fz[3] does not evaluate? Doesn't Mathematica do exactly what you would expect it to do?

By definition fz does not depend on $z$. So I would expect it to evaluate to $y$. But input 18 defines $y$ as $\frac{z^2}{2}$. So after fz evaluates to $y$ this is substituted by $\frac{z^2}{2}$. So Mathematica's result is as expected. I understand that this is not the result you expect. Would you have expected something like this?

ClearAll["Global`*"]
y[z_] := Evaluate[Integrate[z, z]]
fz[z_] := y[z]
fz[3]

This evaluates to $\frac{9}{2}$. By using the delayed evaluations I have changed to order of evaluations.

Cheers,

Marco