An opener of note (3)
I believe the answer is:
key
This clue is a double definition.
'an opener' is the first definition.
(keys open things)
'note' is the second definition.
(thesaurus)
'of' acts as a link.
(Other definitions for key that I've seen before include "It opens the door" , "It can open" , "explanation" , "Scale system for music" , "Small island" .)