He admits being the proprietor (5)
I believe the answer is:
owner
'the proprietor' is the definition.
(synonyms)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for owner that I've seen before include "Holder, proprietor" , "The person in lawful possession" , "One who has something" , "Legal possessor" , "landlord" .)