It is important to place the post function in the correct order of other post functions.
The 'Create' transition is the very first transition, which does not yet has a source status (only destination status - usually Open, but could also be another).
Instead of using the "Clear Field Value" post function in the Create transition, you might consider just configuring no default value for that custom field and don't show it on the create screen.
If you are using the "Clear Field Value" post function in the Create transition, you must put it before the "Creates the issue originally" post function except when clearing labels, which need to be cleared after the issue has been created.
Any other Transition (not Create)
Put the "Clear Field Value" post function anywhere before the "Update change history for an issue and store the issue in the database." post function
A workflow is configured so that the 'Close' transition has the 'Clear Field Value' post-function. The function is configured to clear the 'Security Level' field. If a user closes an issue on this workflow, the value of the 'Security Level' field will be cleared.