Physical activity, but it can be bookish (8)
I believe the answer is:
exercise
'physical activity' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for exercise that I've seen before include "Physical activity done for health and fitness" , "Gym activity?" , "Task set to practise" , "Practice session" , "Exertion to develop fitness" .)