You're missing one small piece in your definition of z. You can't multiply a function handle and a numeric value, but you can multiply the numeric value obtained by evaluating that function handle and a numeric value.
f = @cos;
y = @(x) f(x);
z = @(x) y(x).*sin(x); % Note y(x) instead of y
To test, compare evaluating the function using the function handle and evaluating the function normally, by calling COS and SIN:
v = 0:0.1:2*pi;
usingFH = z(v);
usingFun = cos(v).*sin(v);
isequal(usingFH, usingFun)
That last statement will return true.
