Clue: Drunken bar where diamonds are found? Jewellers made it up (7)
Answer: MIRBANE
Explanation: Definition: Jewellers made it up
BAR* in MINE (diamonds are found in a mine). Chambers says it's "an apparently meaningless word… a name for nitrobenzene, as used in perfumery" – so should the jewellers in the clue have been perfumiers?