AN E145 CREW OVERSHOT ASSIGNED ALT DSNDING INTO PIT.

Narrative

WE WERE CLRED DOWN TO 12000 FT ABOUT 20 MI FROM AIR VOR. UPON REACHING 14000 FT AND ABOUT 5 MI FROM FIX; CTLR CALLED US AND GAVE HOLDING INSTRUCTIONS TO THE CAPT (PNF). WE WERE ABOUT TO GO OVER THE FIX AND WHILE RECEIVING THE CLRNC I DISENGAGED THE AUTOPLT RECEIVING THE AURAL WARNING (AUTOPLT). I DIDN'T HEAR THE CLRNC ALT AND THE CAPT DIDN'T PUT THE NEW ALT IN THE ALERTER; BECAUSE HE WAS TRYING TO GET THE HOLD IN THE FMS. THE NEW ALT WAS 13000 FT. HE (CAPT) NEVER CALLED THE ALT AND I DSNDED TO 12700 FT. THE CTLR ASKED OUR ALT AND THE CAPT THEN ALERTED ME TO THE CLRNC ALT AND I PULLED UP AND HAD THE ACFT AT 13000 FT BY THE END OF HIS XMISSION. THE LATE CLRNC (HOLDING) WAS THE PRIMARY CONTRIBUTING FACTOR TO THE INCIDENT. I (PF) DIDN'T HEAR THE COMPLETE CLRNC DUE TO THE AURAL WARNING AND THE CAPT MADE THE DECISION THAT HORIZ SITUATIONAL AWARENESS WAS MORE CRUCIAL THAN VERT. I WAS TRYING TO FLY THE HOLD WHILE PUTTING IN THE VOR FREQ NOT PAYING ATTN TO THE ALT (NEW) CLRED TO IN THE HOLD. THE RECENT 12000 FT CLRNC WAS RECEIVED SO CLOSE TO THE HOLDING CLRNC I DIDN'T QUESTION THE ALT.