At one time you will appear in the document, he explained (4)
I believe the answer is:
thee
'at one time you will appear' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'in the document he explained' is the wordplay.
'in' indicates a hidden word.
'THEE' can be found hidden inside 'document he explained'.
Can you help me to learn more?
(Other definitions for thee that I've seen before include "Poetically you" , "You in the olden days" , "Old objective second person singular" , "'U'" , "you many years ago" .)