Nothing to write? Make a start! (4)
I believe the answer is:
open
'a start' is the definition.
(thesaurus)
'nothing to write?' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'to write?' becomes 'pen' ('pen' can be a synonym of 'write').
'o'+'pen'='OPEN'
'make' is the link.
(Other definitions for open that I've seen before include "Freely accessible" , "sporting competition" , "begin to > crack" , "Undo; ajar" , "Not secret or hidden" .)