WordNet sense 1 (a chemical compound that sets or fixes something (as a dye or a photographic image)):

fixing agent

WordNet sense 2 (a skilled worker who mends or repairs things):

mender, repairer

WordNet sense 3 (someone who intervenes with authorities for a person in trouble (usually using underhand or illegal methods for a fee)):

influence peddler

WordNet sense 4 (synthetic narcotic drug similar to morphine but less habit-forming; used in narcotic detoxification and maintenance of heroin addiction):

methadone, methadon, dolophine hydrochloride, methadone hydrochloride, synthetic heroin