Defeated Changeling is a mission item received when you defeat a Founder-Changeling in combat. After doing enough damage they will turn into a small puddle that merely tries to run away, at this point you can chase them down and interact with the changeling to capture them.
Item description text as viewed from the duty officer assignment:
- This containment field is holding a defeated Changeling. It churns and bubbles quietly but shows no sign of further resistance.
- Your Security Officer can assist you with delivering this Changeling into the hands of the proper authorities through a Duty Officer assignment. Visit your ship interior to speak with the Security Officer, or access the appropriate assignment from your Duty Officer interface.
- “Jabberwocky”: Defeat Kardek
- “Of Bajor”: Complete "Combat Training: Changeling"; will disappear from inventory after leaving holo-deck