Debt security (4)
I believe the answer is:
lien
'debt security' 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 the property of another" , "Legal right to detain property" , "Sequestration order" , "Right to keep property" , "Legal claim on Nile" .)