Sunday Express initially engaging copy writer (6)
I believe the answer is:
scribe
'writer' is the definition.
(scribe is a kind of writer)
'sunday express initially engaging copy' is the wordplay.
'initially' says to take the initial letters.
'engaging' is an insertion indicator.
'copy' becomes 'crib' (I've seen this before).
The first letters of 'sunday express' is 'se'.
'se' placed around 'crib' is 'SCRIBE'.
(Other definitions for scribe that I've seen before include "Ancient copyist of manuscripts" , "Writer of other days" , "Writer (before printing was developed)" , "Medieval clerk" , "A person writing" .)