Heard Kesey's cruel nurse used mechanical device (7)

I believe the answer is:

ratchet

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'mechanical device' is the definition.
(I know that ratchet is a type of mechanical device)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for ratchet that I've seen before include "Chatter (anag)" , "One-way toothed wheel" , "Chatter about notch on wheel" , "Device forcing motion in one direction" , "Device preventing turning backwards" .)

I've seen this clue in The Irish Times.

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