One in cartel amended clause (7)
I believe the answer is:
article
'clause' is the definition.
('article' can be a synonym of 'clause')
'one in cartel amended' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'amended' indicates an anagram.
'cartel' with letters rearranged gives 'artcle'.
'i' going inside 'artcle' is 'ARTICLE'.
(Other definitions for article that I've seen before include "Piece of journalism" , "Piece in paper" , "Particular object and newspaper report" , "Definite or indefinite determiner" , "Rile cat (anag.)" .)