Running to the teacher is instructive (7)

I believe the answer is:

telling

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'running' is the definition.
Both the definition and answer are adjectives. Perhaps there's an association between them I don't understand?

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

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(Other definitions for telling that I've seen before include "Giving an order" , "of significance" , "Revealing, significant" , "Powerful; relating" , "Counting" .)

I've seen this clue in The Daily Express.

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