Subject of newspaper's petition (5)
I believe the answer is:
issue
'subject' is the definition.
(I know that subject can be written as issue)
'newspaper's petition' is the wordplay.
'newspaper's' becomes 'i's' (UK newspaper).
'petition' becomes 'sue' (both can mean to appeal or ask for something).
'is'+'sue'='ISSUE'
'of' is the link.
(Other definitions for issue that I've seen before include "Send out or distribute for use" , "Problem or concern" , "Bring out - children" , "Sprogs" , "Give or send out" .)