Nothing published including leader of Guardian newspaper (5)
I believe the answer is:
organ
'newspaper' is the definition.
(thesaurus)
'nothing published including leader of guardian' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'published' becomes 'ran' (running is a kind of publishing).
'including' is an insertion indicator.
'leader of' indicates taking the first letters.
The first letter of 'guardian' is 'g'.
'ran' placed around 'g' is 'rgan'.
'o'+'rgan'='ORGAN'
(Other definitions for organ that I've seen before include "Wind instrument found in the body." , "Bodily part or large musical instrument" , "Ear, perhaps" , "ear maybe" , "Wind and keyboard instrument" .)