ACR CREW COVERS CTLR XMISSION WITH READBACK AND DEVIATES FROM ASSIGNED ALT.

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WE WRE HOLDING AT HOLEY INTXN AT 11000 FT. IT WAS THE CAPT'S LEG AND I WAS THE FO OR MONITORING PLT. ANOTHER ACFT WAS HOLDING BELOW AT 9000 FT AND WAS TOLD TO PROCEED DIRECT RBV VOR. WE WERE THEN CLRED TO DSND TO 9000 FT WHILE IN THE HOLD AT HOLEY. AS WE DSNDED; ATC QUESTIONED WHAT WE WERE DOING. WE REPEATED OUR INSTRUCTION TO 'DSND TO 9000 FT WHILE IN THE HOLD AT HOLEY.' SHE CAME BACK AND SAID NOT CORRECT; CLB TO 11000 FT AND GO DIRECT TO RBV VOR. SHE THEN SAID IT WAS HER FAULT BECAUSE SHE DID NOT HEAR US READ BACK THE CLRNC. WE THEN CLBED TO 11000 FT AND WENT DIRECT TO RBV. THE PROB STARTED WHEN SHE INSTRUCTED US TO DSND TO 9000 FT. BEFORE SHE WAS FINISHED WITH HER XMISSION; I READ BACK 'DSND AND MAINTAIN 9000 FT.' I BLOCKED THE LAST PART OF THE XMISSION AND SHE DID NOT HEAR MY READBACK. SHE TOOK THE BLAME FOR THE PROB; HOWEVER; THE LESSON TO BE LEARNED IS TO WAIT UNTIL ATC IS FINISHED WITH THEIR XMISSION BEFORE YOU READ IT BACK. REMEMBERING THAT ATC; WHEN BUSY; IS NOT ALWAYS GOING TO REQUIRE A READBACK OF INSTRUCTIONS OR CLRNCS; AS THEY SHOULD.