I'm not happy with the clue for two reasons - firstly if there's 'no use for' B then the answer should be ARITONES (if I've got no use for green vegetables then I don't have any of them) and secondly the "apostrophe-s" is really doing double duty - because we need BARBITONE'S (with an S) it can't also be an abbreviation for HAS, so the word play translates as 'BARBITONES no use for B'; if the solution were BARITONE, this version would be better: 'Drug's no use for black singer', ie 'BARBITONE has no use for B'.