Project Euler 8: find the maximum product of 13 consecutive digits
Disclaimer: my Forth is extremely rusty.

length does not need to be local; is not a variable, it is a constant. Declare it as such:
13 constant length


Dealing with input. The stack annotation ( a 1000 ) strongly hints that what follows wants to be the word on its own. Indeed, logic should be separated from IO. Consider, for example, something along the lines of
: e008 ( a n -- p)
    ....
;

s&quot; 731671765313306249192...450&quot;
euler008
.

Once the logic and IO are separated, you may use open-file and read-file if you wish.

I do not endorse one-liners, especially if they involve loop. Consider
: e008-multNdigits ( a n -- p )
    1 swap 0
    do
        swap
        dup i +
        c@ [char] 0 -
        rot *
    loop
    nip ;

As a side note, nip is very rarely useful, and usually it is an indication of the suboptimal design. Try to get rid of it. The nipped value, if I am not mistaken, is a base address of the array. I have an impression that its only purpose is to undo a dup in the caller. Try to get rid of both.

The line
  dup maxproduct &gt; if to maxproduct else drop then

is a long way to say
  maxproduct max to maxproduct


Consider having max product at TOS prior to setting up a call to e008-multNdigits. In this case,
  length - 0 do
      dup i + length e008-multNdigits
      max

would suffice, and eliminate the need for the local.



Not Forth-related issues:

The algorithm performs 13000 multiplications. A sliding window approach lets you get away with 1000 multiplications and 1000 divisions. Of course an extra care should be taken when 0 is encountered.

The product of 13 digits may take as much as 42 bits. A naive multiplication fails on a 32-bit cells.



Finally, Project Euler is not about programming. It is about math. To hone your Forth skills, consider implementing classical algorithms, and benchmark them against conventional implementations.