Where is this partition? Are you removing it in order to enlarge an adjacent partition?
The tool for all of this is Disk Utility.
1. Open Disk Utility and select the drive entry (mfgr.'s ID and size) from the left side list.
2. Click on the Partition tab in the DU main window. You should see the graphical sizing window showing the existing partitions. A portion may appear as a blue rectangle representing the used space on a partition.
3. Select the partition you wish to remove by clicking in its space. Click on the Delete [-] button to remove the partition. Click on the Apply button. When the process is completed the partiton will be removed and a gray area will replace it.
4. Select the sizing gadget in the lower right corner of the remaining partition and drag it down to the bottom as far as it will go. Click on the Apply button. When completed the space will be added to the existing partition returning the drive to a single volume.