Install new circlip or wire clip. The wire clip can be a royal pain to install. Try seating one end of the clip in the groove in the piston and "chasing" the rest of the clip into the groove with a screwdriver tip or needlenose pliers.
(If you are installing a circlip in a post-'78 motor, please note that there is a correct orientation for the circlip with respect to the piston: the wristpin circlips have a sharp edge and a rounded edge. The rounded edge goes toward the piston.)