Use anonymous functions, e.g.
f = @(x,y) x + y;
g = @(y) f(4,y);
turns f, a function of 2 variables into g, a function of 1 variable with the other hard-coded. More usefully you can equally do e.g.
where a has been defined ahead of the function definition so is fixed when you actually call g and you just pass in y.
If it is being passed to some other function as a function handle that expects one variable argument, to be optimised then you can do this to have a function of as many variables as you want, where n-1 of them are predefined.