Have you had a new gearbox fitted? The threaded part with the arrow on is a speedo drive. Which part is loose? The speedo drive is held in by the 10 mm bolt about an inch down,the thread then pulls out and has a seal around it,but make sure you pull the plastic black central spindle out at same time. When out,pull the spindle out by the gear at the bottom,there is also a seal on the spindle shaft. When refitting the bolt fits in a hole in the metal housing,mark position of hole with tip ex to ensure it lines up.