FLC OF AN LGT WERE RESPONDING TO A TCASII RA AND HAD LEVELOFF IN DSCNT FOR THAT REASON. WHEN THEY NOTIFIED ATC OF THEIR RESPONSE TO TCASII; ATC ADVISED THAT THEY OVERSHOT ASSIGNED ALT. BOTH CREW MEMBERS HAD HEARD AND SET INTO ALT ALERTER THE ALT READBACK TO ATC AFTER ASSIGNMENT. ATC APOLOGIZED FOR THE MISUNDERSTANDING.

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THANKS FOR TCASII. ON APCH TO SNA; ON THE KAYOE ARR. THE FLT HAD BEEN UNEVENTFUL TO THIS POINT. LEVEL AT 10000 FT WE RECEIVED WHAT THE CAPT AND I BOTH PERCEIVED WAS A CLRNC TO 5000 FT. I; PNF; READ BACK CLRNC AS 'ACR X CLRED TO 5000 FT.' WE STARTED DOWN PASSING 8000 FT. I NOTICED ANOTHER ACFT ON TCASII ABOUT 1 O'CLOCK POS AND 3 MI AT 7000 FT. I POINT IT OUT TO THE CAPT. AT 7700 FT HE SLOWED THE DSCNT AND AT 7500 FT WE RECEIVED AN RA AND LEVELED OFF AT 7500 FT. I IMMEDIATELY CALLED SOCAL AND TOLD HIM THAT WE WERE RESPONDING TO AN RA. HE SAID OUR CLRNC WAS TO 8000 FT. I TOLD HIM WE BOTH HEARD 5 AND I READ BACK 5000 FT. HE APOLOGIZED FOR NOT CATCHING IT ON THE READBACK. I DON'T KNOW IF HE ORIGINALLY SAID 5 OR I HEARD HIM WRONG. THE REST OF THE FLT WAS UNEVENTFUL. APCH WAS VERY BUSY. A GA PLT WAS JAMMING THE FREQ TRYING TO GET AN IFR CLRNC. THE CTLR WAS VERY BUSY. HE APOLOGIZED TWICE BEFORE WE LEFT THE FREQ. IN THE FUTURE I WILL QUESTION CLRNCS THAT MIGHT CROSS OTHER ACFT FLT PATHS. THE KAYOE ARR -- WE WERE ON RADAR VECTORS. BOTH PLTS HEARD 5000 FT ON THE RADIOS AND 5000 FT WAS IN THE ALT WINDOW AS PER SOP.