Start to write on the disc (4)
I believe the answer is:
open
'start' is the definition.
(opening is a kind of starting)
'write on the disc' is the wordplay.
'write' becomes 'pen' ('pen' can be a synonym of 'write').
'on' means one lot of letters go next to another.
'the disc' becomes 'o'.
'pen' after 'o' is 'OPEN'.
'to' acts as a link.
(Other definitions for open that I've seen before include "Release" , "Frank; unobstructed" , "Honest - frank" , "Public" , "unsealed" .)