It may be on the house
I believe the answer is:
'it may be on the house' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lien that I've seen before include "Legal claim on Nile" , "Right to a lawyer" , "Right to retain property until a debt is paid" , "Legal right to detain property" , "The right to take property if an obligation is not met" .)