The Breakdown Hammer (Breakdown Buildable) tool is used to "delete" your buildables from your base. It does not return to your inventory unless you reset- not that you would want to. Breakdown Hammer is a Usable which can be bought in the shop for 10 cash. It has the appearance of the "Delete" hammer found in the default building tools kit.
Using the Breakdown Hammer draws a red line towards the buildable you want to delete, boxing it in a red outline and coloring the buildable red on only your screen. Please note you cannot delete others' buildables with this hammer.
Close-up
The outline
(The icon also changes when you use this to a red hammer with a + sign)