Perhaps Grandma’s to threaten about one item in the will? (8)
I believe the answer is:
heirloom
'item in the will?' is the definition.
'heirloom' can be an answer for 'item' (I have seen 'Valuable item' mean 'heirloom' so perhaps 'item' could also mean 'heirloom'). I am not certain of the 'in the will?' bit.
'perhaps grandma's to threaten about one' is the wordplay.
'perhaps grandma' becomes 'her' (I can't justify this - if you can you should give a lot more credence to this answer).
'to threaten' becomes 'loom' (something threatening might loom over you).
'about' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'her'+'loom'='herloom'
'herloom' enclosing 'i' is 'HEIRLOOM'.
Can you help me to learn more?
(Other definitions for heirloom that I've seen before include "Something that has been in a family for generations" , "Valuable object handed down" , "Family bequest" , "Family artefact" , "Object in a family for several generations" .)