Reveals Stella's distress at not being able to finish (5)

I believe the answer is:

tells

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'reveals' is the definition.
(I've seen this in another clue)

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

(Other definitions for tells that I've seen before include "Narrates" , "Counts" , "Informs, recounts" .)

I've seen this clue in the Evening Standard.

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