FLC OF AN LGT OVERSHOT ALT ON DSCNT DUE TO MISUNDERSTANDING OF THE ASSIGNED ALT AND THE APCH CTLR NOT CATCHING THEIR ERROR DURING THE FLC'S READBACK.

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ON DSCNT ON APCH TO ORD RWY 9L WE RECEIVED A CLRNC TO DSND TO 6000 FT MSL. I; PNF; READ BACK THE CLRNC. I HEARD AND READ BACK 'TURN R TO 160 DEG HDG; SLOW TO 210 KIAS AND DSND TO 6000 FT.' PER COMPANY POLICY I DIALED IT INTO THE MODE CTL PANEL ALT ALERT WINDOW AND THE CAPT (PF) VERBALLY ACKNOWLEDGED AND PHYSICALLY POINTED TO THE ALT IN THE WINDOW. PASSING THROUGH 6500 FT MSL. APCH CALLED OUT TFC AT OUR 9 O'CLOCK POS. WE HAD THE TFC IN SIGHT DURING OUR DSCNT. THERE WAS NO IMMEDIATE CONFLICT FROM OUR PERCEPTION. THE CTLR TURNED US TO A 180 DEG HDG AND ASKED WHAT ALT WE WERE ASSIGNED. I STATED 6000 FT MSL. THE CTLR SAID WE WERE ASSIGNED 7000 FT MSL. IN RETROSPECT I MUST HAVE READ BACK THE WRONG ALT. HE HAD GIVEN US A HEADING; AIRSPD AND ALT CHANGE ALL AT ONE TIME. THERE WAS ALSO A LOT OF RADIO CHATTER AS IS TYPICAL FOR THAT TIME OF DAY AT ORD. THE CTLR DID NOT CORRECT MY INCORRECT ALT READBACK. IN THE BUSY APCH ENVIRONMENT MULTIPLE CLRNCS SHOULD BE AVOIDED; AND IT WOULD HELP IF CTLRS WOULD LISTEN TO PLT READBACKS AND CORRECT THEM IF THEY ARE WRONG. AS PLTS WE MUST LISTEN CLOSELY AND CONFIRM THE CLRNC IF THERE IS ANY DOUBT.