The banana plug that connects to the ST is probably with 3 pieces.
Unscrew the red cap from the lead.
Unscrew the metal ring toward the banana shaped connector portion.
Pull out the red wire from the connector.
Cut off the damaged portion, about 1/2 inch.
Strip the red insulation from the end.
Twist the wires tightly and feed it through the hole on the inside of the connector.
Re-install the metal ring and tighten as much as possible
Re-install the red cap.
Test it by setting the ST to 5 ma, press START, and then touch the leads together. The current should come up to 5.