Control animal that's kicking its hindquarters (4)

I believe the answer is:

rein

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'control' is the definition.
(rein is a kind of control)

I don't understand the rest of the clue.

(Other definitions for rein that I've seen before include "Animal controller" , "Free ... is carte blanche" , "Restriction" , "A means of control" , "Hold back" .)

I've seen this clue in The Independent.

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