PA28 TAXIING FOR DEP AT GVT ENTERED RWY WITHOUT ATC CLRNC.

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THE FBO IS ON GREENVILLE/MAJORS FIELD; ACTIVE RWY IS RWY 17/35; AND THERE IS CONSTRUCTION THAT HAS THE PARALLEL TXWY CLOSED; SO WHEN DEPARTING TO THE S YOU MUST BACK TAXI TO THE DEP END OF THE RWY. I CALLED GREENVILLE/MAJORS TWR FOR TAXI INSTRUCTIONS AND CLRNC FROM THE FBO. TWR RESPONDED WITH 'CLRED TO TAXI TO RWY 17; CALL WHEN READY TO DEPART; HOLD SHORT.' I READ THE CLRNC BACK; AND ASKED FOR CLARIFICATION THAT THE PROPER PROC; DUE TO THE CONSTRUCTION; WAS TO BACK TAXI ON THE ACTIVE RWY. TWR RESPONDED THAT THIS IS THE PROPER PROC. I AM ON THE TXWY TO THE DEP END OF RWY 35 AND UNDERSTOOD THE CLRNC AS CLRED TO TAXI TO THE DEP END OF RWY 17; EXIT THE RWY AND HOLD SHORT AT RWY 17. ABOUT HALF WAY DOWN THE RWY (WHILE BACK TAXING) TWR CALLS AND INFORMS ME THAT I HAVE BUSTED THE HOLD SHORT INSTRUCTIONS GIVEN. I APOLOGIZED FOR THE MISUNDERSTANDING AND CONTINUED WITH CAUTION TO THE DEP END OF THE RWY. THE TWR DID NOT REQUEST ANY DIFFERENT ACTION FROM ME. I FEEL LIKE THE CLRNC GIVEN; CLRED ME TO BACK TAXI TO RWY 17. I BELIEVE THIS WAS CAUSED BY UNCLR COM BY THE TWR. I UNDERSTAND THAT RWY 17/35 IS THE SAME PIECE OF CONCRETE; BUT IF THE HOLD SHORT INSTRUCTIONS WERE INTENDED TO BE UNDERSTOOD AS HOLD SHORT OF THE RWY; THEN I BELIEVE THE CLRNC SHOULD HAVE BEEN GIVEN; CLRED TO TAXI; HOLD SHORT OF THE RWY; EXPECT CLRNC TO BACK TAXI. NOT CLRED TO TAXI TO RWY 17 AND HOLD SHORT.