You can try it like this:
var fieldDescription = jQuery("#dffs_YOUR_FIELD_INTERNAL_NAME .ms-metadata");
Replace YOUR_FIELD_INTERNAL_NAME with your actual field name.
Please note that for this to work, the field description must be in a span with the class “ms-metadata”. This is how it is on SharePoint online, but I’m not sure if this is how it is on earlier versions of SP so you might have to change the selector to find the description element.
If this doesn’t work I recommend you just remove the description from the field and add a table row above the field and add the description there.