He'll admit to being the proprietor (5)
I believe the answer is:
owner
'the proprietor' is the definition.
('owner' can be a synonym of 'proprietor')
'he'll admit' is the wordplay.
I cannot quite see how this works, but
'admit' could be 'own' (own can mean to admit or acknowledge) and 'own' is found within the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'to being' acts as a link.
Can you help me to learn more?
(Other definitions for owner that I've seen before include "One that secured" , "Man of property" , "The person in legal possession" , "possessive type" , "Holder, possessor" .)