Thin core becoming small — it could shock you (5)

I believe the answer is:

taser

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'it could shock you' is the definition.
(a Taser gives electric shocks)

I can't explain the rest of the clue.

(Other definitions for taser that I've seen before include "Police stun weapon" , "The weapon delivers a temporary paralysing electric shock" , "Weapon used to deliver a temporarily paralysing electric shock" , "Shocking device used by the police?" , "Device using electrified darts to stun" .)

I've seen this clue in The Guardian.

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