Hi @Will You
There is no single field level edit permission. But there is a workaround. Remove the field from EDIT SCREEN (Project Settings -> Screens) and then in workflows, create a new transition that returns itself with a newly created screen including this field. Then give condition to this transition.
The following config allows people edit field who match condition in any status:
Or do it for specific status:
Jira doesn't have field level security. The work around is to put the field on the view screen and use a controlled transition to display it or move to the next status if that is your workflow. You say you don't want it to change, but my experience says sometime down the line in some issue you will. So plan for that and put a similar transition somewhere else in the workflow where you might want to change it in the future.