Diagnosis
The clamp spring tension has weakened over time, or the clamp mechanism itself is worn. This is a frequent wear-and-tear issue.
How to Fix It
With the iron unplugged and cold, check the screw/pivot point where the clamp attaches to the barrel. Use the correct size screwdriver to gently tighten it a quarter-turn. Test the tension.
If That Didn't Work
The spring inside the handle is likely fatigued. This is not a user-replaceable part. You'll need a new iron or professional repair, which often costs as much as a new one.
Heads Up
Forcing the clamp or over-tightening the pivot screw can crack the ceramic barrel or strip the screw. Apply minimal pressure when adjusting.