Old priest’s last letter discovered in time (4)
I believe the answer is:
ezra
'old priest's' is the definition.
(biblical figure)
'last letter discovered in time' is the wordplay.
'last letter' becomes 'z' (last letter of the alphabet).
'discovered in' is an insertion indicator.
'time' becomes 'era' (era is a kind of time).
'z' inserted within 'era' is 'EZRA'.
(Other definitions for ezra that I've seen before include "High priest" , "Eponymous OT priest" , "-- Pound, US poet and critic 1885-1972" , "- Pound, twentieth century US poet" , "Book after II Chronicles" .)