B737-300 CREW WAS CLRED TO AN ALT UNINTENDED BY THE APCH CTLR DSNDING ON THE CLARR ONE STAR INTO LAS.

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WHILE DSNDING INTO LAS; LAX CENTER CLRED US TO 'CROSS CLARR AT 12000 FT.' ON ARRIVAL; LAX CENTER ADVISED OUR FLT TO CONTACT LAS APCH ON FREQ 125.6. I WAS FLYING AND THE CAPT CHECKED IN WITH LAS APCH. WE WERE BOTH MONITORING THE FREQ AT THE TIME. LAS APCH SAID DSND AND MAINTAIN 10000 FT AND EXPECT THE VISUAL APCH RWY 25L. CAPT PUT 10000 FT IN THE ALT WINDOW; I VERIFIED THE 10000 FT ASSIGNMENT; AS I CLEARLY HEARD 10000 FT. CAPT READ BACK 10000 FT TO THE CLTR. ONCE WE WERE DSNDING THROUGH 10300 FT THE CTLR ADVISED THAT HE GAVE US 11000 FT. I STARTED TO CLIMB TO 11000 FT AND THE CLTR SAID TO LEVEL AT 10000 FT. THE FREQ 125.6 WAS NOT THAT BUSY AT THE TIME. BOTH CAPT AND I FEEL CONFIDENT THAT THE CTLR ISSUED '10000 FT' EVEN THOUGH HE MEANT TO SAY 11000 FT. THERE WASN'T A TFC CONFLICT. I BELIEVE HE WAS USED TO THE OLD ARRIVAL; THE CRPSO ARRIVAL; WHERE AT WHIGG INTXN LAS APCH WOULD ROUTINELY DSND US FROM 12000 FT TO 10000 FT. HE NEVER ASKED US TO CALL ON THE GND. SUPPLEMENTAL INFO FROM ACN 527881: FIRST OFFICER IS THE PF. WE ARE ON FLT LAX TO LAS FLYING THE CLARR ONE ARRIVAL FOR THE FIRST TIME. AS WE DSND INTO LAS I AM WORKING WITH LAS APCH 125.6. AFTER CROSSING CLARR AT 250 KTS 12000 FT APCH GIVES US A CLRNC TO DSND TO 10000 FT. I READ BACK 10000 FT AND SET 10000 FT IN THE MCP. MY FIRST OFFICER CONFIRMED THE 10000 FT BY STATING ALT AND POINT AT THE MCP WINDOW. SOMETIME AFTER MAKING THE TURN AT SKEBR AND PASSING THROUGH 10500 FT THE APCH CTLR INFORMS ME 'I NEEDED YOU AT 11000 FT.' MY FIRST OFFICER STOPS THE DSCNT AND WE LEVEL AT 10200 FT. I INFORMED THE CLTR THAT WE BOTH HEARD 10000 FT AND THAT'S WHAT WAS READ BACK 10000 FT. HIS RESPONSE WAS 'OK MAINTAIN 10000 FT' THE ARRIVAL WAS CONTINUED WITH NO FURTHER PROB. NOTE: THE APCH CHART FOR THE CLARR DOES NOT DEPICT ANY ALT LIMITS OR MEAS AFTER THE CLARR INTXN. THE TURN AT SKEBR INTXN HAS YOU FLYING INTO HIGH TERRAIN. MOUNT POTOSI 3 TO 4 MI RIGHT OF TRACK ON A 345 DEG HEADING. THE AREA CHART FOR LAS LIST NO HEIGHT FOR MOUNT POTOSI. DURING THE ARRIVAL I DID HAVE TERRAIN WARNING DISPLAYED AND THE ONLY OBSTRUCTION DISPLAYED WAS MOUNT CHARLESTON.