Boss in a criminal organisation opening doors for the mule (6)
I believe the answer is:
donkey
'mule' is the definition.
(thesaurus)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for donkey that I've seen before include "Garment associated with building workers" , "Domesticated ass" , "Braying animal" , "Animal for drudge work" , "dope" .)