Possible Clues:

• Limb: part of the body to be kept in shape
• Member's part is to prepare defence in advance
• A radius is part of it
• Attention in class! Get prepared for action!
• Basis of measure once with listener in class
• Upper limb between the elbow and the wrist
• It's not an underhand drive at court
• Take precautionary defensive measures for part of body
• Figure contains each radius in this
• 57-Across's location