Desire to include grandma in agreement (8)
I believe the answer is:
covenant
'agreement' is the definition.
(covenant is a kind of agreement)
'desire to include grandma' is the wordplay.
'desire' becomes 'covet' (I've seen this before).
'to include' is an insertion indicator.
'grandma' becomes 'nan' (I've seen this in other clues).
'covet' placed around 'nan' is 'COVENANT'.
'in' is the link.
(Other definitions for covenant that I've seen before include "Formal agreement in a convent" , "Promise" , "Formal agreement between two or more parties" , "Contract made in a convent" , "Signed agreement between two or more to do something" .)