China checks URL changes are not singular (6)
I believe the answer is:
plural
'not singular' is the definition.
('plural' is the opposite to 'singular')
'china checks url changes' is the wordplay.
'china' becomes 'pal' (china plate is Cockney rhyming slang for mate).
'checks' is an insertion indicator (inserted letters are checked or controlled).
'changes' indicates an anagram.
'url' is an anagram of 'lur'.
'pal' enclosing 'lur' is 'PLURAL'.
'are' acts as a link.
(Other definitions for plural that I've seen before include "Numbering more than one" , "Composed of more than one" , "More than onefold" , "Form of noun or verb in respect of more than one" , "Consisting of more than one" .)