Writing 'share certificates' at end of document (6)
I believe the answer is:
script
'document' is the definition.
(I have seen 'Orginal document' mean 'script' so perhaps 'document' could also mean 'script')
'writing share certificates at end of' is the wordplay.
'writing' becomes 'scrip' (scrip is a kind of writing).
'share certificates' becomes 't' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'at end of' says to put letters next to each other (some letters go at the end of others).
'scrip'+'t'='SCRIPT'
Can you help me to learn more?
(Other definitions for script that I've seen before include "Written form (of play etc)" , "Written version of a play" , "Handwriting, play text" , "Piece of handwriting" , "Written text of play or film" .)