Granny in desire for agreement (8)
I believe the answer is:
covenant
'agreement' is the definition.
(covenant is a kind of agreement)
'granny in desire' is the wordplay.
'granny' becomes 'nan'.
'in' means one lot of letters goes inside another.
'desire' becomes 'covet' (I've seen this before).
'nan' inserted inside 'covet' is 'COVENANT'.
'for' is the link.
(Other definitions for covenant that I've seen before include "Formal binding agreement" , "Promise" , "Binding agreement between two parties" , "Concordat, compact" , "Formal agreement between two or more parties" .)