Genuine gold article conservationists originally insured comprehensively (9)
I believe the answer is:
authentic
'genuine' is the definition.
(similar in meaning)
'gold article conservationists originally insured comprehensively' is the wordplay.
'gold' becomes 'Au' (Au is the chemical symbol for gold).
'article' becomes 'the' (the definite article).
'conservationists' becomes 'NT' (National Trust).
'originally' suggests taking the first letters.
The first letters of 'insured comprehensively' is 'ic'.
'au'+'the'+'nt'+'ic'='AUTHENTIC'
(Other definitions for authentic that I've seen before include "24 [LEGIT]" , "Real, genuine" , "Ian, the cut is genuine" , "reliable" , "Bona fide" .)