Real gold in that case twinkling endlessly (9)
I believe the answer is:
authentic
'real' is the definition.
(I've seen this in another clue)
'gold in that case twinkling endlessly' is the wordplay.
'gold' becomes 'Au' (Au is the chemical symbol for gold).
'in that case' becomes 'then' (synonyms).
'twinkling' becomes 'tick' (both can mean a short amount of time).
'endlessly' means to remove the last letter.
'tick' with its final letter taken off is 'tic'.
'au'+'then'+'tic'='AUTHENTIC'
(Other definitions for authentic that I've seen before include "Real, genuine" , "Ian, the cut is genuine" , "24 [LEGIT]" , "trustworthy" , "reliable" .)