Servant to be unable to open door having lost duplicate? (6)
I believe the answer is:
lackey
'servant' is the definition.
(a lackey is a servant or assistant)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for lackey that I've seen before include "Sycophant, toady" , "Flunkey, sycophant" , "An archaic servant or servile follower" , "An archaic menial or servant" , "An archaic menial or follower" .)