Tell it's performing well (6)

I believe the answer is:

inform

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'tell' is the definition.
(I know that tell is a more specific form of the action inform)

I cannot understand how the remainder of the clue works.

(Other definitions for inform that I've seen before include "Brief" , "Notify or tell" , "Spill the beans" , "effective" , "Let people know" .)

I've seen this clue in The Daily Express.

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