Mouthpiece heading off on horse (5)

I believe the answer is:

organ

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'mouthpiece' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for organ that I've seen before include "Ear, perhaps" , "Possibly the heart" , "colon?" , "Church keyboard instrument" , "Functional part of body" .)

I've seen this clue in the Irish Times.

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