Genuine gold changes to etch tin (9)
I believe the answer is:
authentic
'genuine' is the definition.
(similar in meaning)
'gold changes to etch tin' is the wordplay.
'gold' becomes 'Au' (Au is the chemical symbol for gold).
'changes to' indicates an anagram (I've seen 'change to' mean this).
'etch'+'tin'='etchtin'
'etchtin' is an anagram of 'thentic'.
'au'+'thentic'='AUTHENTIC'
(Other definitions for authentic that I've seen before include "Bona fide" , "Genuine, not a copy" , "Ian, the cut is genuine" , "Real, genuine" , "24 [LEGIT]" .)