A keeper's right (4)
I believe the answer is:
lien
This clue might be a double definition.
'a keeper's' is the first definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'right' is the second definition.
(legal right)
Can you help me to learn more?
(Other definitions for lien that I've seen before include "Legal claim on Nile" , "Legal right over the Nile" , "Legal claim on the property of another" , "The right to take property if an obligation is not met" , "The right to take property if an obligation is not discharged" .)