Training that may go by the book (8)

I believe the answer is:

exercise

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'training' is the definition.
(I know that exercise is a type of training)

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 "problem" , "Physical training or exertion" , "Keep-fit" , "Employment" , "Task for pupil" .)

I've seen this clue in the Evening Standard.

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