LastName:Left(FullName],InStr([FullName],",")-1)

and, while you didn't ask....

FirstName:Mid([FullName],InStr([FullName],",")+2)

In a query, you would do something like:

Last_Name: LEFT([NameField], IIF(instr([NameField], ",")>1,
instrr([NameField], ",") - 1, LEN([NameField])

If you are certain that every occurance of your [NameField] contains the
comma, you could simply do:

Last_Name: LEFT([NameField], instr([NameField], ",") - 1)

However, both of these use the LEFT function, which will generate an error
if the value in you [NameField] is NULL, so you might want to consider:

Last_Name: Switch(ISNULL([NameField], [NameField], instr([NameField], ",") >
1, LEFT([NameField], instr([NameField], ",") - 1, TRUE, [NameField])

The Switch( ) function accepts pairs of parameters, where the first element
of each pair is a logical statement that will evaluate to True or False.  The
second element is the value that will be returned if the first part evaluates
to TRUE.  In my example above, my first test is to determine whether the
[NameField] is null, if so, I just return the [NameField], but you could put
something in here like "Error: Name is missing".  The second tests whether
there is a comma at position two or greater; if so, it strips everything to
the left of the comma.  The last test TRUE, will always evaluate to TRUE and
is the fall back position if no comma is found.

HTH
Dale