One who admits being the proprietor (5)
I believe the answer is:
owner
'the proprietor' is the definition.
('owner' can be a synonym of 'proprietor')
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for owner that I've seen before include "Title holder?" , "Possessor, proprietor" , "The person in lawful possession" , "Holder, possessor" , "Keeper" .)