Dispenser that's always slipping out of one's hand? (7)

I believe the answer is:

dropper

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But I can't explain it at all! Could you help me understand?

(Other definitions for dropper that I've seen before include "Tube for applying a small amount of liquid" , "One can't catch" , "Medicinal device" , "Small tube with a rubber bulb at one end" , "Dispense with this" .)

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