Old scribe penning last letter in time (4)
I believe the answer is:
ezra
'old scribe' is the definition.
'penning last letter in time' is the wordplay.
'penning' is an insertion indicator.
'last letter' becomes 'z' (last letter of the alphabet).
'in time' becomes 'era' (I've seen this before. I am not sure about the 'in' bit.**).
'z' put into 'era' is 'EZRA'.
(Other definitions for ezra that I've seen before include "good book?" , "Old Testament prophet" , "-- Pound, US poet and critic 1885-1972" , "Old Testament book and priest" , "OT book" .)