Trial meant adjusting will (9)
I believe the answer is:
testament
'will' is the definition.
(synonyms)
'trial meant adjusting' is the wordplay.
'trial' becomes 'test' (synonyms).
'adjusting' indicates an anagram.
'meant' anagrammed gives 'ament'.
'test'+'ament'='TESTAMENT'
(Other definitions for testament that I've seen before include "document" , "New and old parts of the Bible" , "evidence" , "with 1a [NEW] where is Mark?" , "Will - covenant" .)