A PA28 PLT MISUNDERSTOOD CLRNC AND STARTED TKOF ROLL WITHOUT CLRNC.

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WE WERE CLRED INTO POS AND HOLD ON RWY 7. THIS 4000 FT RWY HAS A SIGNIFICANT HILL THAT CRESTS RIGHT IN THE MIDDLE OF IT. THUS; PLANES ON ONE END CAN'T SEE PLANES THAT CROSS ON THE FAR END OF THE RWY. WE'D BEEN IN POS AND HOLD FOR ABOUT 90 SECONDS -- ENOUGH TO MAKE ME UNCOMFORTABLE AND WANT TO QUERY ATC IN CASE THEY'D FORGOTTEN ABOUT US. THERE WAS AN INSTRUCTOR IN THE R SEAT; AS THIS WAS A BFR. THROUGHOUT THE TIME WE WERE ON THE RWY; HE WAS CHATTING QUITE A BIT ABOUT HOW BUSINESS WAS FOR HIM AND FOR THE PLACE WHERE HE INSTRUCTED. THE TWR ADDRESSED A CLRNC TO US. I CAUGHT OUR CALL SIGN; BUT I COULDN'T QUITE MAKE OUT EVERYTHING THE TWR SAID. IT SEEMED TO ME; I ASSUMED THAT THE TWR CLRED US FOR TKOF AND REPEATED OUR CLASS C VFR DEP PROC. THE CFI IN THE R SEAT KEPT TALKING THROUGHOUT THE TWR'S CLRNC. I READ BACK '(CALL SIGN); CLRED FOR TKOF;' AND STARTED TO ADVANCE THE THROTTLE. THE TWR REPLIED THAT WE WERE NOT CLRED FOR TKOF; THAT HE HAD JUST ADVISED US THAT TFC WAS XING ON THE RWY UP AHEAD; AND THAT WE SHOULD REMAIN IN POS AND HOLD. ABOUT 15 SECONDS LATER; HE CLRED US FOR TKOF; AND THE FLT PROCEEDED UNEVENTFULLY. EVALUATION: THIS INCIDENT FITS IN WITH THE INCREASING CONCERN OF LATE ABOUT RWY INCURSION ACCIDENTS. ORDINARILY; I MAINTAIN A STERILE COCKPIT WHEN ON AN ACTIVE RWY; SINCE THAT'S AN UNCOMFORTABLE PLACE TO BE; AND I WANT TO BE FULLY AWARE NOT JUST OF CLRNCS DIRECTED TO ME BUT OF CLRNCS DIRECTED TO OTHER ACFT (EG; TO LAND ON THE RWY WHERE I'M SITTING). HERE; I CEDED SOME OF MY PIC AUTH TO THE CFI IN THE R SEAT BECAUSE IT WAS A BFR. HAD HE BEEN A PAX; I WOULD HAVE TOLD HIM (NICELY) NOT TO TALK WHILE WE WERE AWAITING CLRNC FOR TKOF. I ASSUMED WITHOUT ACTUALLY HEARING THE WORDS THAT ATC HAD CLRED US FOR TKOF; SIMPLY BECAUSE THAT CLRNC WAS WHAT WOULD LOGICALLY BE DIRECTED TO US IN OUR SIT. OBVIOUSLY; I OUGHT NOT TO HAVE ASSUMED THAT; AND SHOULD HAVE ASKED FOR A READBACK OF THE CLRNC. LACK OF FAMILIARITY WITH THE ARPT ALSO PLAYED A PART. I DIDN'T REALIZE THAT OTHER ACFT COULD CROSS THE RWY AHEAD OF ME IN SUCH A WAY THAT I COULDN'T SEE THEM. THE POOR QUALITY OF THE RADIOS IN THE RENTAL ACFT ALSO PLAYED A PART. LESSON LEARNED: MAINTAIN A STERILE COCKPIT WHEN AWAITING CLRNCS TO TAXI ONTO A RWY OR WHEN CLRED INTO POS AND HOLD.