Knee-jerk response to, for example, dropping a mop (6)

I believe the answer is:

reflex

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'knee-jerk' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for reflex that I've seen before include "Bent backwards" , "This action is an automatic response to stimulus" , "Sneeze, blink or hiccup?" , "Movement in response to a stimulus" , "Action not requiring conscious thought" .)

I've seen this clue in the Irish Times (Crosaire) and the Irish Times.

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