Shares the initial writing (6)
I believe the answer is:
script
'writing' is the definition.
(I know that script is a type of writing)
'shares the initial' is the wordplay.
'shares' becomes 'scrip' (I have seen 'extra shares' mean 'scrip' so perhaps 'shares' could also mean 'scrip').
'initial' indicates taking the first letters.
The first letter of 'the' is 't'.
'scrip'+'t'='SCRIPT'
(Other definitions for script that I've seen before include "letters" , "Words of film etc" , "Written version of a dramatic composition" , "Piece of handwriting" , "Orginal document" .)