Journalism in the Guardian, but not in Times or Sun? (7,7)
I believe the answer is:
Here is my best explanation:
'journalism' is the definition.
(leading article is a type of journalistic piece)
'in the guardian but not in times or sun?' is the wordplay.
I cannot quite see how this works, but
'in' is present in the answer.
This explanation may well be incorrect...
(Other definitions for leading article that I've seen before include "Newspaper's comment", "Editorial" and "such as The Times has".)