They lose a quarter of what once belonged to you (3)
I believe the answer is:
thy
'what once belonged to you' is the definition.
I know nothing about this answer so I can't tell whether this works.
'they lose a quarter' is the wordplay.
'lose' is a deletion indicator.
'a quarter' becomes 'e'.
'they' with 'e' taken away is 'THY'.
'of' is the link.
Can you help me to learn more?
(Other definitions for thy that I've seen before include "Belonging to thee" , "Your (archaic)" , "Your, of old" .)