AN ACR E120 FLC MISUNDERSTOOD A CLRNC TO NAV TO THE FIELD AS CLRNC FOR A VISUAL APCH AND STARTED A DSCNT OUT OF THEIR ASSIGNED ALT. THE CTLR ALERTLY ASSIGNED THEM A LOWER ALT.

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I WAS OPERATING IN CLASS B AIRSPACE; DSNDING FROM 9000 FT DOWN TO 6000 FT AND THE FIELD WAS RPTED 'IN SIGHT.' THE CTLR RESPONDED BY SAYING 'PROCEED VISUALLY TO THE FIELD USING YOUR OWN NAV.' I UNDERSTOOD THIS TO MEAN A 'VISUAL APCH.' I DSNDED THROUGH MY ORIGINAL ALT ASSIGNMENT OF 6000 FT. AT 5500 FT (APPROX) THE APCH CTLR SAID MAINTAIN 4000 FT. I THEN REALIZED THAT HE DID NOT INTEND FOR THE PREVIOUS CLRNC TO MEAN 'CLRED FOR A VISUAL APCH.' NO CONFLICT OCCURRED AND THERE WAS NO TFC ON MY TCASII SCREEN. CONTRIBUTING FACTORS WERE THE VFR CLR CONDITIONS WITH THE FIELD BEING CALLED IN SIGHT SO EARLY; AN AMBIGUOUS CLRNC GIVEN BY ATC; AND MY MISUNDERSTANDING OF THE CLRNC. THE ALTDEV OCCURRED OR I SHOULD SAY WAS DISCOVERED WHEN THE DSCNT TO 4000 FT WAS ISSUED AND I REALIZED THE CLRNC FOR A VISUAL APCH WAS NOT GIVEN. CORRECTIVE ACTIONS COULD BE TO ISSUE XING/ALT RESTRS ON VISUAL APCHS (IE; 'CLRED FOR THE VISUAL; MAINTAIN AT OR ABOVE 4000 FT UNTIL 10 DME FROM THE FIELD'); AND TO CLR UP ANY AMBIGUOUS CLRNCS WHEN THEY ARE ISSUED.