BE20 CREW MISINTERPED CLRNC AND DSNDED 500 FT BELOW ASSIGNED ALT.

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ON DEADHEAD RETURN TO APA; I WAS USING THE OPPORTUNITY OF THE EMPTY LEG FOR TRAINING/PRACTICE OF THE SIC WHO WAS THE PF. ON HDOF TO DEN APCH; I REQUESTED A FULL ILS APCH TO APA. ADVISED BY CTLR OF OPPOSITE DIRECTION TO LNDG TFC; AND THAT IT WOULD REQUIRE VFR; TO WHICH I ACKNOWLEDGED THIS WOULD BE OK. AT THIS POINT; I BELIEVE THE SIC THOUGHT WE WERE NOW VFR AND THOUGHT HE COULD DSND. HE WAS IN THE MIDDLE OF BRIEFING THE APCH AND I HAD ASKED A QUESTION REGARDING SAME AND HE WAS SEARCHING FOR THE ANSWER. DURING THE PREVIOUS FEW MINS; THE CTLR POINTED OUT TFC; A NAVAJO; TO WHICH I RESPONDED 'LOOKING.' I DID FIND THE TARGET ALMOST IMMEDIATELY; AND CONFIRMED IT ON TCASII. I DID NOT GET THAT RPTED TO ATC; PRIMARILY DUE TO FREQ CONGESTION. WE DSNDED THROUGH APPROX 14500 FT WHEN CTR CALLED ADVISING OF ALT. MOST OF THIS TIME I WAS MAINTAINING AN OUTSIDE VIGIL FOR TFC AND TRACKING THE NAVAJO. I IMMEDIATELY STOPPED THE DSCNT AND MAINTAINED 14500 FT AND TALKED TO CTR. THE PRIMARY CAUSE OF THE EXCURSION WAS PROBABLY A COMBINATION OF PLT OVERLOAD AND UNCLR COM WITH CTR; PRIMARILY BY ME. THE PLT OVERLOAD I REFER TO IS WITH REGARDS TO THE SIC AND IN A NORMAL SIT WITH A QUALIFIED CAPT I AM SURE THIS WOULD NOT HAVE HAPPENED. AS PIC; I WAS SOLELY RESPONSIBLE FOR ALL ACTIONS -- HIS AND MINE -- AND I SHOULD HAVE BEEN MORE VIGILANT ON MONITORING HIS FLYING AND BEEN ABLE TO CORRECT HIS SOONER. ALTHOUGH I THINK THAT ALLOWING A TRAINEE TO GO FARTHER IN A MISTAKE THAN WOULD BE ALLOWED IN A NON TRAINING FLT; IS HELPFUL TO THEM TO DISCOVERING MISTAKES ON THEIR OWN; I NEED TO BE MORE ACTIVE IN DIRECTING THEM. I ONLY CONDUCT THIS TYPE OF TRAINING FLT IN GOOD WX ON FLTS WITHOUT PAX.