Her rank prohibits one's admission (8)
I believe the answer is:
baroness
'her rank' is the definition.
I can't judge whether this definition defines the answer.
'prohibits one's admission' is the wordplay.
I cannot really understand how this works, but
'one's' could be 'ones' and 'ones' is present in the answer.
'prohibits' could be 'bars' (barring is a kind of prohibiting) and 'bars' is located in the leftover letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for baroness that I've seen before include "Noble" , "Female life peer" , "Noblewoman" , "Peer's title" , "Lady Thatcher?" .)