Genuine gold article, not ‘one carat’ (9)
I believe the answer is:
authentic
'genuine' is the definition.
('authentic' can be similar in meaning to 'genuine')
'gold article not one carat' is the wordplay.
'gold' becomes 'au' (Au is the chemical symbol for gold).
'article' becomes 'the' (the 'definite article' part of speech).
'not' becomes 'nt' ('n't' can be a synonym of 'not').
'one carat' becomes 'ic' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'au'+'the'+'nt'+'ic'='AUTHENTIC'
Can you help me to learn more?
(Other definitions for authentic that I've seen before include "Ian, the cut is genuine" , "trustworthy" , "Real, genuine" , "Genuine, not a copy" , "reliable" .)