They say thin is right (4)
I believe the answer is:
lien
'is right' is the definition.
(legal right)
'they say thin' is the wordplay.
'they say' shows a homophone (sound like).
'thin' becomes 'lean' (associated in meaning).
'lean' sounds like 'LIEN'.
(Other definitions for lien that I've seen before include "Security for a debt" , "Right to a lawyer" , "The right to take property if an obligation is not met" , "Right to retain property until a debt is paid" , "Legal claim on Nile" .)