IN manage security, you can set access restrictions for a given Table, not a given field. Thus, if you want to set up restrictions in Manage | Security, you'd need to put this field in it's own table and link it by a one to one relationship to the original table.
But you can also use a validation rule to prohibit changes to a given field for users with a certain privilege set name.
And scripts with script triggers can also check the privilege set.
The scripted method is not as secure, you have to be careful and consistent with your layout design.
One option - create a duplicate of the layout and remove edit access to all but one field.
Hi David Anders,
I created a duplicate but I don't know how to do the second part you suggested, "remove edit access to all but one field"
Could you point me in the right direction?
On the Inspector's data tab, there's a section labeled "Behavior". Clear the checkbox for Browse mode for a selected field and the user will not be able to modify data in that field on that layout.