Will try, by ten a.m., to be prepared (9)
I believe the answer is:
testament
'will' is the definition.
(synonyms)
'try by ten a m to be prepared' is the wordplay.
'try' becomes 'test' (test is a kind of try).
'by' means one lot of letters go next to another.
'to be prepared' is an anagram indicator.
'ten'+'a'+'m'='tenam'
'tenam' is an anagram of 'ament'.
'test'+'ament'='TESTAMENT'
(Other definitions for testament that I've seen before include "Either of the two main divisions of the Bible" , "Biblical division" , "it could be old or new" , "Witness" , "Will or solemn declaration" .)