A COMMERCIAL INST RATED PLT FLYING A C172 AT MVW FAILED TO MAINTAIN THE ALT ASSIGNED BY ATC.

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WHILE LEVEL AT 5000 FT MSL DURING THE ENRTE PORTION OF AN IFR FLT; I RECEIVED INSTRUCTIONS FROM THE APCH CTLR DIRECTING ME TO THE OM FOR AN ILS APCH. FROM MY STANDPOINT; THERE WAS SOME CONFUSION REGARDING THE INSTRUCTIONS AT THIS POINT AS I HEARD THE CTLR CLR ME TO 'DSND AT MY DISCRETION' AND TURN TO A NEW HDG. I ACKNOWLEDGED THE INSTRUCTIONS 'ROGER; SKYHAWK XXX IS OUT OF 5000 FT AND TURNING 150 DEGS' AND BEGAN MY DSCNT AND TURN TO THE NEW HDG. THE CTLR; BUSY WITH OTHER AIRPLANES ON THE FREQ; DIDN'T ACKNOWLEDGE MY XMISSION. AFTER DSNDING ON THE NEW HDG; THE CTLR CAME ON FREQ AND REPRIMANDED ME FOR DSNDING SAYING I WAS INSTRUCTED TO REMAIN AT 5000 FT. IT WAS AT THIS POINT THAT I REALIZED I MAY HAVE MISUNDERSTOOD THE CLRNC TO DSND; IF THERE IN FACT WAS ONE; AND I IMMEDIATELY STOPPED THE DSCNT. THE CTLR SAID HE COULD GIVE ME A LOWER ALT IN A FEW MINS IF THAT IS WHAT I WANTED AND THERE WERE NO FURTHER XMISSIONS REGARDING THE MISUNDERSTANDING AFTER THIS. THE REST OF THE FLT PROGRESSED NORMALLY AFTER THIS AND I WAS SOON SWITCHED TO ANOTHER FREQ AND CTLR. THE OBVIOUS CAUSE OF THIS WHOLE SIT IS THAT I MISUNDERSTOOD THE CLRNC TO DSND. WHAT FURTHER AGGRAVATED THE PROB WAS THAT THE CTLR DIDN'T CATCH MY READBACK; AFTER WHICH I STARTED AND CONTINUED THE DSCNT. AT NO POINT DID I DSND WITH THE WILLFUL INTENTION OF VIOLATING MY CLRNC CONTRARY TO THE CTLR INSTRUCTIONS -- I DSNDED BASED ON THE INSTRUCTIONS I THOUGHT I HEARD THE CTLR ISSUE. ANOTHER FACTOR THAT CONTRIBUTED TO THE SIT WAS A BUSY FREQ THAT MAY HAVE CAUSED ME TO MISUNDERSTAND THE INSTRUCTIONS AND WHICH DISTR THE CTLR FROM HEARING MY READBACK. THE CORRECTIVE ACTIONS FROM MY PERSPECTIVE ARE PRETTY STRAIGHTFORWARD. OBVIOUSLY; IN THE FUTURE I'M GOING TO MAKE SURE I GET AN ACKNOWLEDGEMENT FROM A CTLR BEFORE I EXECUTE INSTRUCTIONS OR A CLRNC (BUSY FREQ OR NOT). I'M ALSO GOING TO MAKE SURE I UNDERSTAND THE INSTRUCTIONS OR CLRNC WELL ENOUGH SO THAT I CAN AT LEAST WRITE THEM DOWN FOR FUTURE REF -- WHETHER THAT BE A FEW SECONDS OR A WK LATER.