Official proving of a will (7)
I believe the answer is:
probate
'official proving of a will' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for probate that I've seen before include "Formal proof of a will" , "Validation of will" , "Will-checking process" , "Legal proof of a will" , "document that proves a will is genuine" .)