Sunday Express initially taking on copy writer (6)
I believe the answer is:
scribe
'writer' is the definition.
(scribe is a kind of writer)
'sunday express initially taking on copy' is the wordplay.
'sunday' becomes 's'.
'initially' indicates taking the first letters.
'taking on' is a charade indicator (letters next to each other).
'copy' becomes 'crib' (I've seen this before).
The first letter of 'express' is 'e'.
'e' put after 'crib' is 'cribe'.
's'+'cribe'='SCRIBE'
(Other definitions for scribe that I've seen before include "A person writing" , "One writing documents" , "Writer (before printing was developed)" , "A writer of old" , "Mark with a sharp point" .)