Crossword Genius

Boss in a criminal organisation opening doors for the mule (6)

Ross

I believe the answer is:

donkey

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'mule' is the definition.
(thesaurus)

I don't understand the rest of the clue.

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(Other definitions for donkey that I've seen before include "Garment associated with building workers" , "Domesticated ass" , "Braying animal" , "Animal for drudge work" , "dope" .)

I've seen this clue in The Irish Times.
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