An opener can be such a man (3)
I believe the answer is:
key
'an opener' is the definition.
(keys open things)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for key that I've seen before include "//www.crosswordgiant.com/crossword-clue/1252911/" , "Small island" , "Vital" , "Lock-opener" , "Lock opener; low island" .)