Dismantle reins restricting part of Rudolph? (6)

I believe the answer is:

antler

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'part of rudolph?' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for antler that I've seen before include "part of Rudolph" , "Deer-horn" , "Branched animal horn" , "Branched horn of deer" , "Stag horn" .)

I've seen this clue in The Guardian.

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