Will meant to be changed during trial (9)
I believe the answer is:
testament
'will' is the definition.
('testament' can be a synonym of 'will')
'meant to be changed during trial' is the wordplay.
'to be changed' indicates an anagram.
'during' is an insertion indicator.
'trial' becomes 'test' ('test' can be a synonym of 'trial').
'meant' is an anagram of 'tamen'.
'tamen' put within 'test' is 'TESTAMENT'.
(Other definitions for testament that I've seen before include "Written statement of wishes" , "Will or solemn declaration" , "it could be old or new" , "Statement about final intentions" , "Biblical division" .)