SPD RESTR MISTAKEN FOR HDG CHANGE RESULTED IN PLTDEV.

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PREVIOUSLY CLRED TO TURN TO HDG 210 DEGS AT WITKR. I WAS FLYING ACFT ON AUTOPLT. WE RECEIVED ANOTHER CLRNC WHICH THE CAPT ANSWERED AND RESPONDED; BUT I ONLY HEARD HIS RESPONSE 210. APPARENTLY MY THOUGHT WAS TURN TO 210 DEGS. I TURNED THE ACFT AND PASSING ABOUT 240 DEGS; CTLR CALLED AND STATED THAT WE WEREN'T CLRED TO HDG 210 DEGS UNTIL WITKR. I CORRECTED BACK TO COURSE ABOUT THE SAME TIME CTLR ASKED OUR AIRSPD ABOUT 280 KTS. WE WERE SLOWING WHEN I THOUGHT IT WAS TO TURN TO 210 DEGS. THE CAPT DID NOT CATCH THE DEV BECAUSE HE DIDN'T KNOW THAT HE HAD RESPONDED TO THE CTLR. SUPPLEMENTAL INFO FROM ACN 293925: WHILE ON SPICA ARR INTO DTW WE WERE ON THE 070 DEG RADIAL. WE WERE ISSUED A CLRNC AT 40 DME ON THE 070 DEG RADIAL TO TURN TO A 210 DEG HDG AT 10 DME FIX. FO WAS FLYING THE ACFT AND BEGAN A TURN TO 210 DEGS AT 26 MI OUT. AS HE WAS TURNING I REMINDED HIM ABOUT THE 10 MI FIX AND HE INITIATED A TURN BACK ON THE 070 DEG RADIAL. I CAUGHT HIS HDG CHANGE TOO LATE AND FAILED TO PROPERLY MONITOR HIS FLYING.