It is quite pointless - and, as seen above, confusing - for a controller to ever use the expression "radar heading" to a pilot.
If the pilot is instructed to "fly heading 230" or "turn left, heading 230", what more is needed? That is perfectly clear and unambiguous, and if is controlled airspace, is an instruction with which the pilot must comply (other than to avoid immediate danger).
If outside CAS but under RAS, the pilot has already established a verbal contract, his part of the deal being that he will comply with "instructions" (which are actually advisory) until such time as he opts out of the agreement.