The phrase "enough slack" means having an adequate or sufficient amount of freedom, flexibility, or leniency in a given situation or task. Full definition
The necklace should be fitted around the neck, with just enough slack for comfort without pressure but not enough to get even a hand through it. (momtricks.com)
Ensure that you leave enough slack to use the edges as handles when you're removing the bars later. (organicauthority.com)
I measured according to their directions, and I have a perfect fit, with just enough slack to allow for a wee bit of weight gain / loss. (theprofessionalhobo.com)