Exercises requiring introductions of an extensive range of pens (8)

I believe the answer is:

aerobics

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'exercises' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for aerobics that I've seen before include "Keep-fit exercises" , "Exercise regime" , "Form of exercises" , "Strenuous exercises" , "Form of exercise" .)

I've seen this clue in The Independent.

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